Your looking the wrong place, look in browse.ini, specifically this section:
# Action used when assigning a role to a user or user group
[AssignRole]
StartNode=users
SelectionType=multiple
ReturnType=ObjectID
Class[]
Class[]=user
Class[]=user_group
TopLevelNodes[]
TopLevelNodes[]=users
(Selection type is probably single on your install, you can change it if you like)
I changed the browse.ini file in the override folder. I cleared my caches. I added classes identifier. The "SelectionType=multiple"parameter is working but I can't select my news classes. I am limited to user and user_group