Did you create the map with Galaxy Forge? If so then this the first reply (second post) in this thread may be of interest:
http://www.TinyURL.com/Sins5
Basically, you need to email the map file to your friend and have him put it in the right place. It's actually very easy if you know where the right place is, which is the hard part.
If your map is a "Map Designer"-made map then it should just auto-download to everyone who joins the game.
