Groups can currently not be deleted as such.
However, the way to achieve deleting a group is to go to group settings and select Leave.
The reason why the delete option is not currenlty present, is that this would also delete the group from all the members of the group.
In case you do want to achieve deleting the group and removing all members' access to it.
- First remove the members access from the group
- Then leave the group
You can remove members from a group if you're the admin of the group. Just click on their profile image in the ribbon and select remove member.