[TigerC10]

EuroBeat2, you need to create a usergroup for every discussion area that you want to be private. Then, when you want to grant access to someone for that area, make one of their additional usergroups into the usergroup for that discussion area (so that their primary usergroup stays the same).

PB Usergroup Choice On Registration is a mod that will help you do this at the time of registration. You will have to fuss with the code yourself to get it to do what you want - or ask the mod developer how to change it such that additional usergroups are supported... But at least it's getting somewhere.