Dear Tijs ,
Thanks for your reply ,First of all I solved this issue i have no problem in assign user or group to task
problem i need to make two buttons in the form one for save only and still in the same user private tasks and another one to save and send to next user in the process .
i did it by activitiService.getTaskService().setVariables(taskId, expectedValues); and not use
formService.submitTaskFormData(tradingForm.getTaskId(), expectedValues);
because submitTaskFormData method will complete this task.