How do you do that without switching away from the list of the Groups though? When I open the Address book, I can't see the list of Groups all of the time...ughh. IS there a way to see it like it was in Tiger? That made sense to me.
Go to your listing of groups in the address book (Menu: View\Groups); this listing will stay on the left. Do a search on the card you wish to add; the card will appear on the right. Drag the listing into the group on the left. There may be more elegant ways, but this works for me.