Thanks.
In the code I start the server like this:
taskService = new TaskService(emf, SystemEventListenerFactory.getSystemEventListener());
taskSession = taskService.createSession();
taskSession.addUser(myuser);
server = new MinaTaskServer(taskService);
and I invoke it only once - startup of application.
so you mean that I can write taskSession.addGroup(myuser); while the server is running will add it to db?