[Alexis17]

Generally guests that are part of an opening act it's in the agreement and sign-offs aren't as cumbersome, as they already have them as part of the opening act contract, or they are friends and recurring. Random sit-ins are much harder. Things like James Brown, John Mayer or Joshua Redman guesting are different than Bela Fleck, Jeff Coffin, Warren Haynes (when he was opening with Gov't Mule or now randomly). It's a lot harder than a new cover which is more or less rubber-stamping.