In the group editing view you can use "All" or "Group" buttons at the bottom of the page, to switch between showing all available cameras, and group only cameras:
A status message will show you how many cameras are in the list, and if there is a filter in effect.
To apply a filter to the list, press . See Filter Camera List.
When showing all cameras (“All” is selected), use the star at the right of a camera to add or remove it from the group.
When showing group cameras (“Group” is selected), use the star at the right of a camera to remove it from the group.