Here you go mate, the list of upcoming Group Builds up until 2013(!):
Aug 1st-Nov 30th, 2010 - Battle of Britain
Nov 1st, 2010-Feb 28th, 2011 - Commonwealth / Night Fighters Split Build
Feb 1st-May 31st, 2011 - VVS/Eastern Front 1941-45 Split Build
May 1st-Aug 30th, 2011 - Focke Wulfs Messerschmitts
Aug 1st-Nov 30th, 2011 - Aces Aircraft of WW2
Nov 1st, 2011-Feb 28th, 2012 - Aircraft in Foreign Service WW2 / Captured Aircraft SPLIT BUILD
Feb 1st-May 31st, 2012 - Your Favorite Aircraft of All Time
May 1st-Aug 30th,2012 - Home Country Modern Aircraft / Spitfire Marks SPLIT BUILD
Aug 1st-Nov 30th, 2012 - Aircraft Carrier Planes
Nov 1st, 2012-Feb 28th, 2013 - From WW1 to WW2 1914-39
Feb 1st-May 31st, 2013 - The Jet Age 1944-45 / Recon-Transport-Observer Aircraft of WW2 SPLIT BUILD
Battle of Britain GB is currently under way and it ends on 30th November (each GB lasts for 4 months). As for entering current GB all you need to do is to start a thread in Modeling Section of the forum for appropriate GB where you'll be posting pictures of the progress of your build.
For a current GB you can do it here:
#7 Battle of Britain - Aircraft of World War II - Warbird Forums