A similar situation where If I have Process Instance key/business key and Group and I need to figure out list of Tasks then I guess following piece of code should work -
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ProcessInstanceQuery piQuery = new ProcessInstanceQueryImpl();
ProcessInstance pi = piQuery.processInstanceKey(businessKey).list().get(0);
// find the Task by process instance id and user id - since the task is assigned to user group
// user associated with group will also be candidate
TaskQuery tQuery = taskService.createTaskQuery().processInstanceId(pi.getId()).candidate(userId);
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what do you say guys?