Go to the Admin section of your group using the Admin link at the bottom of the page. Once you have reached the Admin page, you will see several things that you can manage.

Look for the Admin Players section, and click on the Manage link for it. You will see a list of players registered to your group.

For any user that you want to promote, click the "Promote Player to Admin" link next to the user.

To remove admin privilege from click the "Demote Admin To Player" link next to the player you want to demote.