1. Login to Web-based interface (https://www.myibay.com/) if you are not logged in yet.
2. Create a group in the "Groups" (this is the button on the top).
3. Add your auction item.
4. Click "Snipe" button on the left.
5. Select your group in "Group" pull-down menu.
6. Click "Change Group".
If you use Firefox extension then you need to use steps 1 and 2 only. You will have an option to change a group while you are adding an item.
Please feel free to comment if you have more questions or need more information.