Once you have claimed your brewery page, you may add 'Managers' to this page from your Account Settings.
In order to add a Manager, they must already have an Untappd user account.
Managers have the same access to brewery pages as administrators, but through their Untappd user account. In order for a Manager to access the brewery page, they must search for and select the brewery, then hit 'Use as this brewery?' at the top of the page.
The Manager is now impersonating the brewery and can access the Beer Management tool from the drop down.
This is only available on https://untappd.com and not the Untappd mobile app.