Drawing straws would be ok... but it's always fun to actually make teams. What we did was pick captains and the captains had to build a team then.
Other than preparing teams, it's always nice if everyone pitches in to buy food and drinks for the event. Make like a $5 per person entry fee and it all goes towards food and so on. Possibly a cheap trophy???
I'm guessing it's going to be on private land correct? If it's a woodsball tourny than your fine or you could build a feild. It's up to you.
One this we did was make a set of rules for the tourny to try to prevent cheating and so on.
IF u are wanting othier people to come put a flier up but if it is just 20 to 30 u could make 10 threeman teams of 6 teams 5 players on each team and make sure u have Co2 and air fill up and make sure for plenty of paint make sure other people bring paint