Attribute Cases only within a User Group

Is this possible? It currently shows all users on the system in the drop down

If this is in regards to the referral/transfer/assign functionality, you could set up your user groups the same as you do agencies, and when you use a referral or transfer functionality, then when a user selects a specific service and an agency, then they only see the users within that agency.

If this is in regards to the filters, these are by design set to display all users within the system.