WSUS GPO enabled but still can't see computers....

I have just installed WSUS and have enabled GP and set that all computers are
pointed to http://server:8530/ but they are still not showing up in the WSUS
admin page under computers and obviously can't create any groups without
computers. What do I have to do in Group Policy to make the computers show
up. I want to be able to create groups to test the updates first before
rolling them out to all the computers. I tested the connections from one of
the client computers using http://SERVER/selfupdate/au/x86/xp/en/wuaucomp.cab
as it says to do in the documentation and it works fine. Any insight would be
greatly appreciated.