Great question, Witherspoon!
The people who had a reputation for rhythm were Mike and Vicky who were keen dancers. That’s how they met, if I recall.
I think Lillian would be the least inhibited dancer because she’s always game for a laugh and free-spirited. And Linda would give it a go – she’s shown she’s got rhythm already: she’s good on the bongos!
Nearly everyone else would be trying to be cool or would be totally self-conscious and unconvincing. Especially the younger cast, who’d probably not want to dance unless they were all dancing and then once they got started it would just look like a punch up in a nightclub.
I reckon there’s not one of the Grundy’s who could find anything called rhythm even if the whole lot of them started searching high and low.