Use the "brains" cheat (see the all cheats list below).

Then once zombies start spawning, shoot bullets around to get them to run after you (around 30 will do).

Then run to a store that has your crew outside and get one to follow you.

Make him need to be revived by shooting him then heaps of your crew should start spawning.

Note however that they won't all follow you as the limit is three.