Requirement: To add another admin to an existing group, you must already be an admin of this group.

First go to “My Groups” using the drop-down menu in the top right header.
Select the desired group from the group overview here.

At this point you should be able to view the group's member list. Next to each name you will see three dots on the right side. If you click on this, you can add the user as an additional admin using the “Make admin” option.