I have had numerous users in the last couple weeks tell me that their group panel disappears and in a panic they don't know that "CG" will close the group without save and "FG" will finish the group. We have been using Revit 2011 since around when it was released so I don't know if this is a Service Pack 2 issue or my users just were not using groups as much until recently.
I DO HAVE A FIX THOUGH!
The fix is relatively easy, if you get your panel back then you can start the edit group command. Once edit group is activated you can click on the modify tab and drag the edit group panel into the ribbon. So far this has worked for anyone who had the issue.
If you cannot get your Edit Group panel back try the following: turn off the properties window and the project browser window. Turn them back on and try edit group again.
David Light also sent me some info that he found if one starts either a new project or family, then go back to original then you should get the panel back.
Either way, once you get it back... DOCK IT! No need to go through hoops over and over when trying to edit your groups.