I’m sorry, i didn’t explain my self, and forget my english by the way. What i want to do is that, when a given user wants to mention another she can only see those users that are on the groups wich have access to the category the user is posting into. I think that, for now, i would be happy to limit the users search to only those that are on any group that the current user is, but the need is to scope the search to all the users which belong to all the groups that has access to the category the current user is posting into.
I think this would be really hard to come by, any help will be much appreciated.