Got a quick update from Ryan Key from Yellowcard about the guest-vocals on the new album. Basically, the guest vocals are part of a huge group four-part harmony in the bridge of a new song. It includes Ryan, Alex from All Time Low, Cassadee Pope, and Tay from We Are The In Crowd. It's a four part harmony in the bridge, not lead vocals. Having spent some time with the band throughout the years, I feel relatively confident in saying this is the most excited I've ever heard them about a new batch of songs.
I think I would've rather heard them separately on a few tracks since they vocals will probably blend in with each other during a gang chorus, but either way it's pretty cool.