So how is it up in the arctic circle?
Seriously, I don't think Nintendo was intending for you to buy 3 other GBAs so your friends could join in. The GBA to GCN ratio is large enough that this should satistically be an easy setup. If you can't find people, then tough, don't buy the game. If you can though, it'll probably one of the best gaming experiences that you'll ever have.
Also, these games can actually save you money. Only one person has to actually buy the game.