select u from UserGroup ug inner join ug.user u where ug.group_id = :groupId order by u.lastname
As a named query:
@NamedQuery( name = "User.findByGroupId", query = "SELECT u FROM UserGroup ug " + "INNER JOIN ug.user u WHERE ug.group_id = :groupId ORDER BY u.lastname" )
Use paths in the HQL statement, from one entity to the other. See the Hibernate documentation on HQL and joins for details.