Of course! There's barely need to limit the player numbers in an RP of this simplicity. The barbarians are just about to happen upon the forest clearing, there's no need to wait for me to 'declare' their official arrival. 

We are all GMs, and we are all players in this. That is the strength of freeform! My only real authority as creator is to step in and slap people if they do things that upsets everyone else :)