You should have an option at groups[at least in most of the ones I used was like that]. Usually at the groups you've a list of the groups available and then an edit for each one where you set the posts to auto-change title and what not, the icon for them, if you want them to be with moderator power/admin power/just be a user with a cute name after X posts/etc.
Ofc I already used some that didn't had that option available so maybe check/ask that at the support forum of that free board is better unless someone already used that type and know where it's that[not really wanting to make a new forum there just to check where is that located].