I think it is wonderful when a group has multiple owners. So much easier, working together, to keep the content fresh and interesting. To add another owner to a group:
- Have the person join your group.
- Select Manage > Members in the upper right of your group homepage.
- After the person's name and in the Role column, select the change link.
- From the drop-down list select Change Role to Administrator.
- Click on OK.
Do read the Help Center document, All About: Spaces and Groups, and please join the new Group Owners group to share ideas with other group owners for maintaining a group and growing its membership.