Ive done a little testing of my own.

Im doing the group changes in Firefox
and a user account using IE 9

The changes update imedaitly on the next screen change as the user.

But !
I have a member posting at noon when the group changes were made at 10 am. I did contact this person and they confirmed that they did log in and out. So i'm now thinking it could be browser dependant (Not sure).

Work in progress.

That sounds plausible! It seems unlikely to matter which browser you were using to make changes (I was using FF), but when they take effect may depend on the user's browser/settings, possibly?

FF has been falling down, imo. Lastest update improved things but it still has probs, I think.