I found a seller that had a cell phone I wanted. I created a group to buy 1 cell phone. There were 15 identical listings that I created snipes for within the group. Each listing ended about 30 minutes (not seconds) apart. I won the first auction, however, the balance of the bids were not cancelled as they should have been and I ended up with 12 items. I've used groups before without this problem. Did I do something wrong, or did your Group function operate incorrectly?

The problem is that your limit for the Group was set to win "all". If you need to limit by 1 item only, then you should put "1" as your limit.
You can manage Groups setting by clicking "Group" button on the top toolbar.
If you always need to create your future Groups with limit "1", then you can change the default in "Profile" section (the "Default 'to win' for Group" field).

Please also check the following FAQ section on how to use Groups just in case if you need to refresh it:
http://www.myibay.com/login/faq/2/#q11

For your situation I could suggest to either cover seller's fees and ask him to cancel the rest of the auctions. Or re-sell those phones back on eBay.