Here's a quick video that shows you how to make someone a Groupsite Manager as well as how to transfer Manager status from one member to another.
How To Make Someone A Groupsite Manager from Shaun Callahan on Vimeo.
And, here are brief written instructions:
How do I make someone else a Groupsite Manager?
You must be a Groupsite manager to enable this permission for another member. Click on the main-level MANAGER tab and choose the "Manage Members" sub-tab. You will see a list of members and columns of related information. Locate the member whose status you want to change and then click the entry in the "status" column (third column). A new page will load for that individual and you change and save the member's status from "member" to "manager."