I didn't make that up! :P
If there are 400 people on Tengaged, then your odds of having 1 person from your last game join are 1/382 (in the ideal situation). Your odds decrease with each person that joins by 1/x+1. So, having another person join from your other game would be 1/383. You continue to do that until you get to 1/400, the 19th person from your other casting. So, you take 382 x 383 x 384 x 385, etc and you get that answer. :D