Since jBPM5.4 the UserGroupCallback mechanism (http://docs.jboss.org/jbpm/v5.4/userguide/ch.human-tasks.html#d0e5419) is used behind the scenes to get the groups of a user. There is no need to specify the group/s in the client call anymore since you are already specifying the actor.
It is still an open question how do you retrieve the tasks belonging to a list of groups without caring about any particular actor.