American Express Platinum cards issued in the USA provide access to:

• AA lounges when flying AA, or codeshare partners if ticketed as AA
• DL lounges when flying DL
• US lounges when flying any airline
• Airspace lounges when flying any airline
• Priority Pass Select which includes AS lounges, hundreds of independent lounges, but specifically excludes all UA lounges
• $200 Airline Fee Credit can be used to partially reimburse the cost of a United Club membership. There are many other ways to claim the $200.

AA, DL, US, Airspace lounge access includes spouse and children under age 21, or up to two traveling companions, at no cost.
Priority Pass access includes only the cardholder. Guests are $27 each per visit.

I believe the Ritz Carlton card includes only the Lounge Club membership, which provides access to only a subset of the Priority Pass network.