I can answer them, and you can see this in action in the small demo with jBPM. For jBPM the user/group are just a string called actorID, nothing more, nothing less. For showing up tasklists, the jBPM example webapp uses the user that is logged in tyo show his tasks. If there is also a group a user belongs to, you have to wrtite some code to also show these tasks so a user can assign one to himself.
Transitions are defined in jBPM and yes you can retrieve them. Look in the webapp examplecode how this is done. Realy, look at it, you'll learn a lot.