[Redirected] - Skate 3 Online - Can't join friends

Hey everyone I'm wondering how I can play Skate 3 with my friends? I downloaded the game onto my console, and they bought the disc and I'm unable to join them. However I can play with my friends that also downloaded the game, but they also cannot play with our friends that bought the disc. Please help, I don't want to have to go and buy the disc.