Kampervan,
By "encouraging the audience to join in" the club has already circumvented any 'rules' by making it common practice to sing along with Gordeanna, or anybody who turns up (I could add Kevin Mitchell, Len Graham, Sheila Stewart....... and dozens more, but that would be just my personal preferences).
It really is far safer to (if you are to have it at all) leave the decision to the main singer, though personally, I'm always perfectly satisfied if there are enough chorus/refrain songs.
Jim Carroll