Just a thought:
A guest pass would be really cool. Invited members only would be placed into a guest pass group where they could do a bit more than a normal guest, and then if they decided to actually join they could get a credit of some sort or be placed into a special group. Just a thought because I was thinking this would be cool so that people could get a temporary, limited feel of your group through invites, without actually being considered still a "guest".
And since your trusted members (I hope you trust them!) would be referring them, giving them access to try out things would be cool. And after their guest pass period is up, all of their activity would disappear or remain, depending on if they actually registered or not.
I'd actually love to see this feature.