How to encourage teamwork

How do you make community members want to enlarge their communities? (having communities that require a mix of chars, or a critical number of the same type, is one way)

Ok by having three basic classes it will encourage players to help one another.  I have tried to make the classes such that they will be complimentary.  Scouts will be able to move about quickly to locate enemy players, and they will be needed to hack enemy equipment that the group comes across.  I feel that there should be a cost if one is frozen by the enemy and is not able to be revived by another play, and so when a player is frozen and teleports back to the closest teleport point they will drop everything in their inventory.  Dropped items can then be picked up by the enemy, meaning that going out solo is going to be not so smart.  Before the enemy can utilize the dropped equipment a scout will need to complete the hacking minigame for each item, but because of a limited pack space they will not be able to carry very much.  Soldiers are the damage output of any mission.  They have a larger pack and will be able to utilize a larger assortment of weaponry, but they won't be able to hack equipment nor will they be able to gather the most valuable resources unless they take them from enemy players.  Builders are going to be slow and therefore they will be easy targets of they are out alone, but they will be the only class able to heal others and structures, utilize landmine type weapons, and collect the resources that the other 2 classes cannot.  Also builders will be able to do the most damage to structures, will have the largest pack space, and I also think that builders will be the only class that comes equipped with a light source automatically for underground exploration.  I realize that players will still be able to go solo as each class which I wouldn't have any other way, but they will be complimented by having the other classes there, and the more the merrier, especially when venturing far from the base.