A basic part of many movies, books and Generals missions is freeing prisoners.
In this little map you will free some prisoners (have their team join yours) by entering the fence they are in.

You can easily use other triggers to make the teams join, for example when you have killed a third team (the guards), or when you have enough money, etc.

Try the map, and then take it apart in WorldBuilder to see the scripting.

Get map, zipped