Is there any way through Madden to see if anyone from the league is on or do I need to add everyone in the league as friends?
No they don't need to be your friend, but I think it helps.
Either way, when you're in madden, go to Online -> Online Leagues -> My Leagues
Once you're into your league check League Standings (or something like that). It will show everyone in your league with their win/loss record, the team they are using etc. If you scroll all the way over to the right, you'll see a circle. If that circle has a check that means they are online.
Another, (and better IMHO) method: Click R3 in the standings page. This will bring up your message center. Use L2 to scroll to your address book. Then us R2 to scroll to your league's members. This is better b/c it will tell you not only if they are online, but if they are playing Madden and what in Madden they are doing (Franchise, online game, front end, etc). If you see someone who isn't in an online game already, hit 'em up with a league game invite!
I've also found that it is best if you let someone know that you are inviting them through the PSN messaging. If just seems to do a better job of getting their attention than just inviting through Madden.
Hope that helps. Have fun!