Good move! BWii has been underated, and is a worthy addition to gamers' collections.
There are three online options - Co-op (basically playing through a level together to capture enemy HQ), Assault (one player has to capture the other player's assets) and Skirmish (kill as many of the other side as possible) - Assault and Skirmish are counter op.
As soon as you boot the game up you can join in online battles - although you don't have access to all levels (you need to unlock a good portion).
Its worth noting that online is extremely competent, i've experience only a couple of games drop out, there is absolutely no lag that i've seen. Very impressive.
I've made a thread where members have left their friend codes here. Do remember to PM people when adding them to BWii they'll need to add you back- and don't forget to add me!