How do I delete a group?
I did a search, and came up with two results, the first for the web interface, the second for Windows app.

When I click on a group and bring up the contextual menu, their is no delete item and the delete button in the toolbar is grayed out.

Mac version does not support Groups management yet.
You will have to use the Web interface to delete and manage Groups for now. The Web interface supports all modern browsers on Mac OS X (Safari, Firefox, Google Chrome, Opera) and accessible through the following link: http://www.myibidder.com/