To delete and hide groups you must access the menu of the 3 vertical points and select the option Edit group. From this modal window you will have to press the Actions button where you will see the options Delete and Hide.
Deleting a group will erase all data from it: students, their marks, planning, events, annotations, etc. Once deleted if you synchronize from your devices, this group will be deleted both on the web and on the devices and you will not be able to recover it again.
If instead of deleting, what you do is hide the group, in this case you stop seeing the group and all its associated data, but these will not be deleted, they will only be hidden.
To show the hidden groups again, you must go to the group list screen, and from the 3 vertical points menu select the Show hidden groups option. Then you can select a hidden group in the list to make it show again following the same procedure you did to hide it.
You can also store a group in the cloud.