How do I disband or delete a group?
While there's no explicit mechanism for simply killing a group while it has members, a group owner can still disband a group by kicking everyone else out, making sure nobody new can join the group, and (optionally) leaving it himself. After 48 hours with a membership total of one or zero, the group is disbanded automatically.
Unfortunately, this page has no information on a rather pressing related question which is the re-activation of defunct groups. One can assume, by reason of the fact that this information is *not* mentioned on this and on several other pages where the topic would normally be discussed, that Linden Labs actively dsicourages the practice of re-activating groups. But it woudl be nice if they actually said so, or at least made the information available somewhere else.
It's relatively certain that groups get de-activated by mistake all the time, and to have definitive information on the topic in the knowledge base woudl be a good thing for all concerned.