"...just sit there like dumplings with
no idea what to do. Am I supposed to come with a chord chart written out in felt marker
and hold it up?"

Well, if you want accompaniment, you'll have to provide whatever the other player(s) might need. An ensemble can only play to the level of its least skilled. But you may not want accompaniment. If you want to be the "star", and to hell with the rest of the group who are merely basking in your glow, well then, let 'em all figure it out for themselves. Playing in a session, playing with an ensemble is, after all, all about you (the fiddler). :^)