You can remove groups by pressing the 3 dots and clicking on remove group
The history (attendance report, trainings etc) is still available though you remove the group.
You can not remove group that have scheduled events in the future.
The schedule icon next to the 3 dots shows that this particular group has events in the near future. If you hoover over the schedule icon you can see what events are coming up.
If you want to remove a group that has future events you must cancel all the events for that group before removing.