GP Racing
You can still unlock everything in multiplayer, there are to types of GP races. You can be on a team or be seprate, there can be up to 4 players. The best way to do it if you have 4 players is to be on seprate teams, try to help your freinds out to get in first place. This is a great skill.

Turbo Boosting
Turbo bossting in multiplayer is 2x faster than in single player. To do this "special" boost, both players must press (A) at the same time when it says go. Try pressing (A) as fast as you can when it says 1, this gives you a better chance of getting the double boost.

Battle Mode
You can't unlock anything in this mode, its just for fun. The point of battle mode is to hit the other player when you get something from the item box and shoot it at them. If they have the shine sprite, take it from them. You will be the leader, but watch out for the other players!