BuddyPress.org

How to add separate user role’s all users list in drop down menu of custom add profile field of Buddy Press ?

For example I have three types of roles. Admin,Doctor,Patient. I built Patient registration form. In this form I add the field using Buddy Press plugin’s sub menu as using below steps…

First I go to Users->Profile Fields->Add New Field->
Here in Name I add a name of field label “Assigned to”
Under the Type I select “Drop Down”
Now I stuck. I want here I need users list of my all users whose role is Doctor. In another words I want here my sites all registered Doctors list.

Is anyone have an idea to do this ?

Regards and Thanks in Advance