Come on down
The catchphrase used to invite audience members to the stage to become contestants in game shows.
This, as you might expect is an American phrase. It has long been used when making an invitation, especially in advertising when trying to attract people to a store or an event. Here's an example from a December 1901 edition of the New Jersey newspaper, The Trenton Times, in an advertisement for Dunham & Co. Department Store:
"Come on down and take in women's garments coats and suits and fur jackets and fur capes..."
This idiom was picked up later by game show hosts when inviting contestants to come down out of the audience and join them on stage, always accompanied by whoops of delight. This was best known from 'The Price Is Right' game show, which began in 1972, hosted by Bob Barker. There was an earlier version of the show dating from 1956, but I can find no record of the phrase being used in that. 'Come on down' became a catchphrase for the programme, which is still running in the USA (2006).