Define which users can be selected in the custom "Users Field" Merged

The "Users Field" custom field is useful for when we want to identify the key members of a project team. However, because we have over 500 Aha users, when someone clicks on the field to select a user, there are far too many options available to pick from. It would be great if we could identify groups of users, such a Project Managers, Product Owners, etc, and then the Users Field could be edited to indicate what type of users should be included in the options. That way if the custom field is for Project Manager, clicking on that field only allows a user to choose one of the users who is a Project Manager.