The T&Cs say:
A valid United Club membership card or one-time pass and a government-issued photo ID (such as a driver's license, passport or military ID) are required.
Membership cards are nontransferable and may not be loaned to others. United representatives will confiscate membership cards that are expired or presented by anyone other than the member.
United Club members may bring their spouse and dependent children under 21, or up to two guests, into any United Club location. Members must accompany their family and guests during their United Club visit.
A one-time pass admits one person, with no guest privileges.
United reserves the right to limit club access to members traveling on same-day tickets.
United reserves the right to limit access for one-time pass users during high-traffic times when United Club locations are at or near capacity.
So it would have to be under a "reserves the right" event rather than a general case.