How would I go about making a team balance script?

I have completed my game (Yay for me) but I have a problem. The teams dont balance, there was 1 round where it was 8 vs 0. No one went to the other team.

The current system I use is it teleports players to the map, and when they touch the part they teleported to it sets there team depending on what side of the map there on. But it doesnt actually balance the teams so is there any way you can balance teams?