I have my forum setup so that new members who come in are automatically in a prospect group. After 25 posts they are automatically added to a "full member" group with more privileges. The problem i'm having is that there seems to be no way to change their primary user group automatically. Only the secondary user group seems to be able to be altered with User Group Promotions.
Is there a way around this? As membership grows, this will be a problem because I will have to update users manually to their correct groups.
PLEASE forget the way you did it under myBB, vBulletin, SMF, phpBB or whatever you were using. ALL users have Registered as their primary as has been indicated above. You then do all else utilizing secondary groups (which are kind of a misnomer since if you have 5 additional groups that they are in, are they all secondary? ).
All you need to do is set up promotions that once they meet a certain requirement (say X number of posts and Y days of membership) the promotion runs and ADDS them to that secondary group.
I'll just add my support to the above responses. You have to throw away any pre-conceptions about how to go about promoting users. All users, from brand newbies right up to Super Admins should be in the same primary group with a base set of permissions. You add to these permissions incrementally by placing members into more and more secondary usergroups - each one has a "delta" of extra permissions over the one below it.