Just hold down the Apple key, then click and drag the group name to switch the order around.
Click the group name by itself to minimize the group and hide the screennames, that should help view the groups (especially if you have a lot of buddies)
The only thing I can think of is that you hit the limit on contacts. (...Though I don't even know if Adium has a limit - maybe the AIM or MSN or whatever service does.)
...On second thought, the Adium FAQ says that while Adium has no limits, AIM, ICQ and .Mac all have 500 contact limits. I don't know the others.
To move contacts, note that you only need to hold down command if you have a borderless contact window.