Reading the CLG guidance documents or ADB might be your best bet.
http://www.communities.gov.uk/publications/fire/firesafetyrisk7 <-CLG Guidance
http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADB2_2006.pdf <- ADB Vol 2, check section 3
Or for a history lesson, Post War Building Studies no 29, Fire Grading of Buildings.
I have to agree with Retty (Not that I don't like agreeing with Retty of course) and lean towards it not being much of a useful skill on its own. It is just part of the bigger picture. The calculations are not difficult enough to warrant any sort of course, as IMO a reasonable tutor should be able to explain it within an hour including worked examples.
Dumbing it down a bit:
[Expected maximum occupancy] = [floorspace of room] divided by [the area people usually take up in that sort of occupancy (floorspace factor from ADB)]
[Necessary exit width in mm] = [Expected maximum occupancy] multiplied by [5]
With this exit width in mind, then you need to think about losing exits to fire, inward/outward opening doors, fastenings on doors etc
With existing buildings you work backwards, you already know the exit width available, so you take off the worst case exit(s) lost to fire, then find out how many can escape through the remaining doors using values from ADB or CLG guidance. (i.e. a 1050mm door allows 220 people through according to ADB) This gives the number of persons that the room can safely accommodate.