There's more of a strained relationship there. When they left, Josh penned a nasty exit statement that completely trashed the band. He and his family (not including Zac) have also made some questionable (pro-republican, anti-gay) statements in the past and fans are well aware of that, so I think most are adverse to the idea of him joining back at any point. Though, they do seem to be on speaking terms; Josh and Hayley follow and like each other's tweets...for whatever that's worth.
During their recording of S/T they spoke about having "guest writers" come in to collaborate. If you look up their blog videos they posted back then- I'm sure you'll see it. Tons of bands do this. I remember even hearing about A Static Lullaby having some people come to help them write. I think it was for Faso? Can't remember but yeah. It's not uncommon.