Maybe we could create two team designations.

Competitive teams would be required to have the same number of members as each other. The general idea is that these teams are in it to win it and anyone who wants to be part of a winning team should join one of these teams.

Relaxed teams would be for members who just want to participate here and there and have fun. These teams might win (but probably won't) and won't be capped by member limits. There will be no expectation of performance or victory and, thus, no stress or pressure on the members.