Edit: Wait! It's fun to play against a team when there is a 2:1 ratio? Even an, one or two more ppl, advantage can be a huge factor. In my experience, anyways. 
I was exaggerating for effect. More practical would be for a 2 or 3 person difference. The reason for that existing at all is because I could see something going on where people are just joining and happen to pick the same side at once through no fault of their own.
Another thing I came up with is that if numbers reach some certain unbalance threshold people could be forcibly reassigned... but honestly I don't like that very much because I don't see a good way to do it. Personally I would be pretty steamed if I had been playing on a side from the beginning and working hard to secure a win, and then because a bunch of people came in late and joined the 'winning' side I am force-moved to the losing group.
Don't see that you can do too much about people rage-quitting (or leaving for any other reason). It isn't like you can FORCE them to keep playing.
Also, I recently thought of another method to handle this, rather than hard coded rules go in through game mechanics. Give multipliers to the money earned by the side with fewer people and have it ramp up really fast. It would probably be best to have this be based on the overall ratio of players in addition to the absolute gap. Maybe have it affect point accumulation and ticket usage as well.
That way new people joining the 'winning' side end up tipping certain factors in the favor of the people who are getting stomped on. They are getting points faster for the same actions, they are getting more money, allowing them to buy bigger things, and when they die their team tickets don't drop so much. Obviously this would require a lot of tweaking to get working properly, but it seams feasible.