The ID of the model res.users is an integer. So you could load the list and select one using a python random function, see for example https://docs.python.org/2/library/random.html.

But maybe you would rather use the model hr.employee to make sure you choose a real employee and not admin or sales or whatever :-)

Better you use some another strategy instead of random selection because you could end up with overloaded users while others with nothing to do. I suggest a round robin strategy for that

Internal lottery :)

chosen = fired!