How can I add or remove group leaders?

If you are a group owner, click on the "Members" tab in the group and search for the person you want to add or remove as a group leader from the group.

Once you find them click on the "moderate ∨" button next to them. From that dropdown menu you will be able to add or remove the member as a group leader.

Note: only one member can be a group owner, but you can have many group leaders.