Allow monitors to be in multiple groups

I didn’t realize it at the time but with the groups, a Monitor can only be part of one group at a time. I had thought that a monitor could be part of multiple groups at a time, which I really would have liked. I had thought this because when you create a group and start typing names of a monitor to add to the group, it allows us to select any group, including ones that are already part of a group. Because of this, the existing membership of a monitor in a group will get removed without warning. Since any monitor was available, I had thought the monitor would be added to a 2nd or 3rd group name, but it wasn’t, it was just removed from the existing group.