Sorry, "them" is Premium Global Assist, and no, you don't have to enroll in anything. It's a benefit that comes with the card.

Here's what the "PREMIUM GLOBAL ASSIST® HOTLINE1" PDF document says about the medical evacuation benefit:

"Emergency Medical Transportation Assistance
In the event that the Card Member or another covered family member (your spouse or domestic partner, dependent up to age 23, or age 26 if full-time student) traveling on the same trip itinerary as the Card Member becomes injured or ill while traveling and is seeking or has sought medical treatment, the Premium Global Assist Hotline medical department can assess the medical need for transportation and provide the service.

A medical evacuation may be provided at no cost to the Card Member or covered family member from point of illness or injury (when the Card Member or covered family member is under the care of a local medical service provider or facility) to a more appropriate medical facility or to a hospital near the person’s home as determined by the Premium Global Assist Hotline designated physician. The Premium Global Assist Hotline designated physician, in consultation with the local medical service provider or facility, will determine whether such transport is medically necessary and advisable. The event must be within the first 90 days of the trip and cannot be a pre-existing condition. A pre-existing condition is any sickness, illness, or injury that has manifested itself, become acute, or was being treated in the 60 day period immediately prior to the start of a trip. The person needing evacuation may need to complete a medical information release as required by the Health Insurance Portability and Accountability Act (a “HIPAA Release”), or provide authorization for next of kin to complete the release. Emergency transportation services in connection with the medical emergency may also be provided to a covered family member pursuant to the full Emergency Medical Transportation Assistance Terms and Conditions at the end of this document. Subject to additional important terms, conditions and exclusions. Please see full Terms and Conditions at the end of this document.

Very Important: Any costs for medical transport not authorized and arranged through the Premium Global Assist Hotline Program are solely the responsibility of the Card Member and such costs will not be reimbursed. Premium Global Assist Hotline does not cover medical expenses (with the exception of cost incurred during the transport) nor transportation of personal possessions including luggage."

Here are the terms and conditions referred to above:

"Premium Global Assist Hotline - Emergency Medical Transportation Assistance
Description
The following emergency services are provided to Card Members (including Additional Card Members) with Cards providing Premium Global Assist Hotline, where (a) such Card is on an Account that has not been suspended or canceled, (b) Premium Global Assist Hotline was an existing benefit of the Card Account at the beginning of the trip, (c) the Card Member is traveling on a trip of 90 days or less, and (d) the Card Member is 100 miles or more from the Card Member’s American Express U.S. billing address. Additionally, coverage is extended to each of the following members of the Card Member’s family who are traveling on the same trip itinerary as the Card Member: the Card Member’s spouse or domestic partner, and dependent children under the age of 23 or under the age of 26 if enrolled in school on a full-time basis.
Access
Access to the following services is available 24 hours a day within the United States and Canada, via toll-free telephone call to 1-800-345-AMEX (1-800-345-2639), and from outside the United States and Canada via a collect telephone call to 1-715-343-7979 or via fax to 1-715-345-1219.
Please note that any assistance provided by this benefit cannot be in violation of U.S. economic or trade sanctions. In addition, assistance is not provided for travel to Cuba.
Services
Premium Global Assist Hotline (“PGA”) shall provide, at no cost except as specifically noted herein, the following emergency services to covered Card Members and family members:
A. Emergency information will be provided via telephone by a PGA-designated physician or assistance coordinator on the PGA staff, when the Card Member is unable to get assistance locally. Information will include the names, local addresses and local telephone numbers of the nearest medical service provider, such as hospitals and clinics; attorneys; or other providers of emergency services such as U.S. consulates or embassies, as appropriate and as dictated by the resources available in each country or location. Wherever possible, such information will be provided on two or more emergency services providers. No guarantee is made as to the quality of the emergency service provided, and the final selection of the provider, and any risk, shall be the sole responsibility of the Card Member.
B. Medical transport and evacuation will be provided, at no cost, to the covered Card Member or family member, to another, more appropriate medical facility, or from point of origin back to a hospital near their home, when the covered Card Member or family member is under the care of a local medical service provider or facility and when the PGA designated physician, in consultation with the local medical service provider or facility, determines that such transport is medically necessary and advisable due to inadequacy of local facilities. Eligibility for medical transport is contingent upon the covered Card Member or family member not 1) traveling against physician’s advice, 2) traveling with a pre-existing condition*, or 3) traveling to seek medical treatment. Decisions made by the PGA-designated physician and the local medical service provider as to 1) the medical need for evacuation, transportation and/or repatriation services, 2) the means and/or timing of any transportation, medical equipment and supplies and medical personnel to be used in connection therewith and 3) the selection of the Card Member’s final destination, are medical decisions based on medical factors, and as such, are beyond the control of the PGA Program and shall be conclusive in determining the need for such services.
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Very Important: Any risks and costs for medical transport not authorized and arranged through the PGA Program are solely the responsibility of the Card Member and such costs will not be reimbursed. PGA does not cover medical expenses (with the exception of approved costs incurred during transport) nor transportation of personal possessions including luggage.
C. Medical assessment, coordination and monitoring of treatment of the covered Card Member or family member will continue, by all reasonable means, where medically advisable, until the Card Member or family member is released from treatment or is sent home. These steps will include an initial follow up by the PGA-designated physician with the local medical service provider or facility to discuss the preliminary diagnosis and evaluate the recommended treatment, and a contact by the PGA-designated physician or coordinator with the local medical service provider or facility, and/or the Card Member, as frequently as every other day throughout the hospital treatment period to monitor the Card Member’s or family member’s progress for the purpose of determining the adequacy and necessity of the treatment being provided.
D. PGA will handle billing for medical care (outside of approved costs of transport) as follows:
1. When the Card Member has indicated to the PGA administrative coordinator that he/she has health insurance that would cover the services in question, the administrative coordinator on staff will first ask the local medical service provider or facility to bill the Card Member’s insurance company and, if the local medical service provider or facility is unwilling to do that, will ask them to bill the Card Member directly, rather than requiring the Card Member to pay on site.
2. When a local medical service provider or facility refuses to provide needed medical services to a covered Card Member or family member without being guaranteed an advanced payment for such services, the PGA Program will, with the Card Member’s authorization, endeavor to obtain such payment from the Card Member’s family or other individuals indicated by the Card Member, or, with the authorization of the Card Member, will guarantee such payment and, if necessary, advance such payment up to a limit of U.S. $5,000 and charge the Card Member’s Account for the payment amount, subject to charge authorization by American Express.
E. Other emergency transportation services will be provided to covered Card Members in a medical emergency:
1. If a Card Member is traveling alone or with a child 16 years old or younger, or if a traveling companion who is not a family member has been required to leave the Card Member, the PGA Program will arrange and pay for round-trip, economy-class transportation for a member of the Card Member’s family to travel to the place of treatment, if it is reasonably anticipated that the covered Card Member will be hospitalized for ten consecutive days or more. The PGA Program will not be responsible for the cost of the family member’s meals or accommodations.
2. If any covered children who are 16 years old or younger are left unattended by the Card Member’s injury or sickness, PGA will arrange and pay for one-way economy class transportation to return them to their principal residence. (Any additional costs are the responsibility of the Card Member.) Should it be necessary for an attendant to accompany the children, PGA will make arrangements for a qualified escort to accompany the children at no cost to the Card Member.
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3. If a decision is made to evacuate a Card Member to another medical service facility, PGA will arrange and pay for one-way, economy-class transportation for one other covered family member traveling with the Card Member to the Card Member’s principal residence, provided that the other family member’s ticket has become invalid as a result of the Card Member’s illness or injury.
F. Emergency message transmission will be made through the best efforts of PGA on behalf of, and at the request of, the sick or injured Card Member to the Card Member’s immediate family, next of kin, and a reasonable number of business associates.
G. In case of death of a covered Card Member or family member while traveling, PGA will provide the necessary administrative services to effect the transportation of the mortal remains of the covered individual back to the Card Member’s principal place of residence or place of burial, whichever is closer, and will pay all administrative and transportation expenses, and, subject to PGA’s approval, the cost of a coffin or other encasement of the remains suitable for travel.
Note: The above services may be limited or prevented by act of God, war, civil commotion, labor disputes, unavailability of goods or services, or refusal of permission by local authorities. If you have questions about Premium Global Assist Hotline, please call the Customer Service number on the back of your Card.
Important Instructions
• Always have your Account number ready when calling PGA.
• Please note that PGA must arrange all medical and covered family member’s transportation. PGA will not reimburse you for expenses you incur on your own.
• Whenever possible, call collect from overseas. In some countries, long distance calls may require operator assistance, and collect calls may not be possible in some countries.
• When dialing direct, give a phone number and exact location (hotel, city, street, etc.) where you can be reached.
*A pre-existing condition is any sickness, illness, or injury that has manifested itself, become acute, or was being treated in the 60-day period immediately prior to the start of a trip.