[Suggestion] Custom field type: User - filter

Hi,

It would be great if you could limit the lists of users to specified user groups, and not just static users. For example I would like to create a user group called "customers" and then be able to select the correct customer (user) based on membership in that group. This would be more dynamic compared to adding each customer user individually. 

This feature would scale well for assignee as well, as you could create teams and only be able so select users based on membership in those teams.