This will find all Users where the field is empty (not exists) aka users where no roles are added

-_exists_:userRoles.userRoleId
CODE

This will find all Users where the field is not empty (exists) aka users where roles are added

_exists_:userRoles.userRoleId
CODE

This will find users of a specific role. 

userRoles.userRoleId:49c5052617c849cf95de3a98d6408f4a
CODE

The ID is shown in the URL in the Role Detail View. 
VIDEO: How To Search with Query for User Role