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. To do this, log into your account (hover over your user icon in the top right, switch account to use the listing you would like to manage), and then click on the “Manage your listing” link. Once you see the interface like in the screenshot below, please use the manager's tab on the right-hand side of the screen to enter the user's email address or Foursquare user ID into the search field, and then click "Find." Once you find an account with whom you would like to share the manager's access, please use the “add” button. This will share the manager's access with another Foursquare user, so please ensure you trust the person to whom you are granting access to your location.