Filter user in Select (bpm:assignee) control (Alfresco)

Question

I have bpm:assignee control in my custom wokflow, and that's ok. But wright now user can pick up all alfresco users for bpm:assignee. I wan't to limit user so that when he goes to select control and enter some term for search in search result appear just user from specific group or just some users defined by some criteria (it means that user can not search over entire alfresco user database, just over specific set of users). Does anybody know how i can achieve this in my custom workflow?

Regards, Aleksandar