Help: How to Play Online

[NoD]

Just got RockBand and PS3 on Christmas and I set up a PSN account.

So being that I have never played any game online, I'm trying to figure out how to play RockBand online:

I'm not at the PS3 now but looking for help so I can try it tonight.

Yesterday I went in the community menu and only saw scoreboard and stuff.
Went to multiplayer and did not see any online options.

Total newbie, so any advise would be great,
Do I invite friends 1st over the PSN so they are on my PS3 friends list?
How do you find out who's playing RockBand, and ask them to play?
Can you just watch people play online?

Also if there is another thread with all this info feel free to link it?

Thanks!