Who is joining who? They already have a sound their looking for. You’re going to a battle of the bands type of thing, right? It seems, if you want to experience this offer, you for now, should be making them happy. Standing around between songs at jams, play some of your stuff see if it catches on.
Right now its about the next gig for them, they think you can make them sound better if you add what they are looking for. Do you want to do that?
They are already paying you a complement offering to let you play, meet them halfway and play their tone......... for now.