Hello everybody, interesting discussion going on here
I agree with this Mandy - there is no throw away line in the film (or in the story). This really shows that the 2 men do not understand women aand are not very interested in them. Randall's "don't know", looking totally unbothered, speaks volumes.
To me, it is almost like a code, Jack responding to Randall's earlier glances and prodding to see if he got it right. And Randall gets the message and feels encouraged to make his invitation. An invitation which would look innocent enough without the 'you know?' at the end