Pretty much anything Pete Seeger, he made a ton of old religious hymnals into union songs that are fun for call and response acoustic guitar sessions. Also anytime they say union men or whatnot, just change it to union folk
Edit: Also this might be out of left-field but another type of song thats really fun to sing in groups is sea shanties. Here's my favorite: Roll the old chariot. I don't know any shanties with explicitly labor lyrics but you can kinda change the lyrics (e.g. "Oh a fair contract wouldn't do us any harm")
