I started a group that was disbanded, so I tried to start another one with the same name, and I can't.
For technical reasons, it's not possible to have two groups with the same name, even if one of them isn't active. You'll have to pick a new name for your group (you might try substituting numbers for alphabetical characters in time-honored 1337 tradition).
Unfortunately, it's our policy not to alter the names of disbanded groups in order to accommodate requests for new versions of them. This is because it's a relatively labor-intensive process for us, and if we decided to start doing it, we'd be too busy chasing down disbanded groups to do the other things we'd like to -- such as answering support tickets, fixing bugs, creating new features, and generally making your Second Life experience as excellent as possible.