In my opinion, no audience member should ever join in the verses uninvited - it is ill-mannered and can be destructive.
I want to hear the singer's interpretation of a song, not anybody else's, whether they know (or think they know) the song, which is what a performance should be about.
A few U.K. singers have earned themselves bad reputations here in Ireland for this practice - one particular very fine singer who really should know better has the habit of audibly humming the tune if she doesn't know the words - utterly crazy.
Some club members have been known to call out "One singer, one song" when it happens - maybe a poster on the wall or a tee-shirt is in order.
I would also suggest to singers that they think very hard before they invite audiences to join in verses - especially guests who have been booked on the basis of their own, and nobody else's singing
Jim Carroll