Once you're managing a listing, it's simple to add other managers!
Listings can be managed by both user accounts and Foursquare brand pages. Use Foursquare as your listing (hover over your user icon in the top right, and then click the listing you'd like to add a manager to), and then click on "listings" link under "Manage your locations".
You should see a "Managers" box on the right side of the screen.
Enter in the user's email address or Foursquare user ID into the box, and then click "Find".