I think it was possible in the old bf to have more users in the group than slots, but only by invite from the leader.
So the match was hidden at the world map when there are enough players in the group, even if they didnt select a slot yet.
Of course it should always be possible for the leader to select a slot though.
There actually have been numerous updates already since the open stress test started. But it is not easy to find the cause and fix all current problems, but the devs are working on it as fast as they can. Please be patient.
Did you try to reinstall DirectX 9, .net frameword and c++ redist?
http://download.microsoft.com/download/8/4/A/84A35BF1-DAFE-4AE8-82AF-AD2AE20B6B14/directx_Jun2010_redist.exe
http://www.mediafire.com/download/66qcju6wesb0i6u/GearboxRedist.exe
So you dont get any error messages anymore? What exactly does happen? Are you able to start the updater but not the game, or not even the updater? Did you try to open UAC.exe?
Note that you will need DirectX 9 specifically. http://download.microsoft.com/download/8/4/A/84A35BF1-DAFE-4AE8-82AF-AD2AE20B6B14/directx_Jun2010_redist.exe
Please try to make a screenshot next time or at least type over the exact error message. You can make a screenshot with the snipping tool that is included bin windows.
Could you also post your log file when this happens please? Make a copy of it before you restart the game since else it will overwrite.