You will need at least 5 survivors on the survivor board, so make sure to complete Help Wanted first. You will also need to have upgraded the first three tiers of both the Mechanic and Smarts skills. This will allow you to have the +2 Maximum Posse Size that both of the skills have. The best thing to do is wait until you have 5 survivors and then recruit them from the survivor board into your posse; that way you don't run the risk of them inadvertently dying.