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.
