Thanks for your response JE.
I am implementing my own application to work with JBPM. It seems that from your answer, and from my knowledge so far on JBPM, that user-group assignment restrictions are outside the scope of JBPM.
In your experience, have you used any sort of Rules (Drools) to determine if a User is authorised to perform a task? If so, is there any simple example you can point me to?
That would be very helpful.
Also, I will have a look at the UserGroupCallback class.
Thanks
Charlie B