You can register UserGroupCallBack via UserGroupCallbackManager. Or may be you can use the DefaultUserGroupCallBackImpl which can be created based on simple java.util.Properties files that has user and group mappings e.g.
alex=manager,sales,admin
After that you can use the method taskClient.getTasksAssignedAsPotentialOwner (String userId, String language, TaskClientHandler.TaskSummaryResponseHandler responseHandler)
This will invoke the callback and will collect tasks considering all the groups of the userid, for instance for alex it will find tasks for all three groups (manager,sales,admin).
Regards.