Entity Reference Filter: Filter users by role I want to attach managers to site customers. I made Entity Reference "user-to-user" and now I want to select users only with role "Managers".

How can I make such filter?
As far I understood you want to see only "Managers" in Entity Reference widget on user add/edit page. 

Entity reference widget can filter values by using "Views" module.

You can just create an "Entity reference view" and get all users with role "Manager" there.

Here is instruction for widget settings: https://drupal.stackexchange.com/a/38917/6309