Record yourself a number of times to emulate a crowd of people. Record yourself from different angles and areas of the room, cover mouth and record, hold your nose and record. After u've got your takes go and pan, EQ, distort and add reverb... do what you need.
I did a similar on one of my tracks, i shouted the word "drums" from about 6 diff places in my bedroom, panned each take, added a lil reverb and there it was, sounded bloody great... lol