I think it would be helpful to create a membership tier for people who are interested in joining, but want to see what the place is like first. Open sign-ups would reduce barriers to participation and increase our visibility (so we are listed in the directory at joinmastodon.org).
One way it could work: anyone can sign up. After a period of time (e. g. 3 months), they can apply for membership, after which they are asked to donate and can vote on decisions. Not sure what should happen if they don't want to join - if their account gets muted/deleted, or if there's an "inactive" status, or what.
I'd be interested in hearing your thoughts on this.