A group's Founder and group Admin(s) have the ability to make other members Admins for the group.
To promote a member of your group:
Navigate to the group's main page.
Click on the "Members" link, located below the group's cover photo.
Click on the orange "Edit Members" button.
Locate the member You wish to promote.
Click the orange "Promote" button, located beneath the member's username and avatar.