Attendees adding to number in such bars? (7)
I believe the answer is:
karaoke
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for karaoke that I've seen before include "Pub singing (night)" , "Amateur singing to recorded music, perhaps in a pub" , "Musical entertainment, originally from Japan" , "Entertainment in pubs where customers sing to backing track" , "Amateur singing entertainment" .)