You can share the administrative responsibilities for your group between multiple group members.
A user must first be a member of your group before you can give them administrative responsibility.
View your group's homepage. In the actions/menu block that is in the upper left hand portion of the page click on the link that shows the number of subscriptions in the group. This will take you to the subscription administration page.
Next to each user that is not already an administrator is an option called "admin: create". Click that link to give the user shared admin responsibility for the group.
If you are the group creator, you can never remove your own administrative rights to the group.