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.

In app:

If you want to edit og remove the group you click on the Subgroup.

Then you click on the three dots in the upper right corner and then you can click on Message group, Edit group or Remove group. Then you remove the age group.