@dansup another way to approach it is to set the user population limit as a multiple of the number of moderators. As the number of people formally acting as moderators goes up, more registrations can be allowed. For example, 150 users per moderator, although different numbers could work depending on the character of the instance. I guess you'd still want to set a hard limit though, for cases where you get a rash of moderators volunteering and then quitting too fast to be replaced.