Our club asked about multi-group memberships flying at charter club fields. The AMA never forbad members of the other groups from flying. The legalities of the charter only required that for an AMA chartered club to allow flying at their field, the flyers had to be AMA members. They could have any other insurance or membership they wanted. If any club kept SFA, or UMA or any other group out, it was their own choice, not the AMA's.
This decision was communicated to us from 2 different club officers talking to two different AMA officials. We were concerned about it, and for a time, were even looking at chartering through SFA. They didn't seem to meet our needs.