how to: toggle access to a restricted forum for a specific forums member?
 

Apologies in advance for the long-windedness, just trying to be clear:

We have a section of our site that is for mods and admins only, let's call it 'X-Files'. Occasionally we want to share a limited number of the threads in 'X-Files' with one or more other members, these would be from a specific usergroup called 'Business People'. Normally 'Business People' cannot see 'X-Files'.

What I thought I would do is create a new forum, say 'Issues', for the threads that were to be (temporarily) shared. Then I would create a new usergroup called 'Access to Issues' with permission to see the 'Issues' forum. The intention is that only admins, mods and 'Access to Issues' members could see the 'Issues' forum.

So, by (temporarily) assigning 'Access to Issues' as an _additional_ usergroup to a member from 'Business People' the idea is that they would then have access to the 'Issues' forum. When they were done in 'Issues', I would remove 'Access to Issues' as an additional usergroup from their User record in the CP and they'd return to being a normal 'Business People' member.

Well, it doesn't work. Assigning 'Access to Issues' as an additional group to a 'Business People' member does not give them access to the 'Issues' forum even though that forum's permissions have been edited to say that 'Access to Issues' members should see 'Issues'. It's as if the Primary usergroup permissions ('Business People') is over-riding the Additional usergroup's permissions ('Access to Issues') such that the Additional has no (apparent) effect: 'Business People' are blocked from 'Issues' normally and they remain blocked when they are (additionally) assigned 'Access to Issues'.

I guess the question is (a) what am I doing wrong?, and/or (b) what should I be doing to get this type of thing working?