Algorithm for Soccer Team Building

[xbeanx]

I'm hoping someone can suggest an algorithm for creating soccer teams based on some criteria..

1) Players suggest 2 or 3 'friends' whom they would like to be on a team with.
2) Teams would be evened out by player weight.
3) Players suggest their favorite position, and would be assigned to teams that need such players.

Can this problem be reduced to some known algorithm? I think I've encountered this kind of problem in the past, but cannot remember how I solved it. I would prefer an optimal solution, which is why I ask here. I know I can do this, but I'm not sure on an optimal solution. An optimal solution would be one where all friends are on the same team, each team is close to the same weight (or average weight) and each player plays his/her favorite position. The closest to these rules I consider optimal.

Thanks for any suggestions..
Greg