[Trinket]

You would like to mantain their group so they create files with the same group as xbian? right?
What I did in my setup is have all the processes that I want to be stopped executed by the group "download" but I added that group as a secondary one for each user so that when they create files they do it with the same user as Xbian.
It's posible to add them by name, you should modify the "startall.py" and "stopall.py" files so that they stop processes by name.