So as far as I understand it is possible to build your own maps, is it possible to play these maps in multiplayer?

One more question: Is it possible to be on the same team with a friend againt a computer?

yes its possible to play with your friend against PC, you need host it as melee pick pc start game and then after game started go to "alliance" and ally/vis your friend

if you make your own map, player who will join your game will download it, you will see numbers 1-100 when player reasech 100 you will be allowed to start game