Limit Requestor, Cc, and AdminCc to just users in the group?

Is there a way to limit the Requestor, Cc, and AdminCc autofill to just users with valid email addresses in the group?

Right now user “Bob” in group “My Company” starts typing “Bill’s” email address into the Cc text entry box and every “Bill” in RequestTracker populates the autofill.

Request Tracker 5.0.1