Group of people including a man and a woman playing instruments on a double-decker red Planet Hollywood sightseeing bus.
A group of people including a man and a woman are playing instruments on a double-decker red Planet Hollywood sightseeing bus.
Sentence 1 of people playing instruments on a double-decker bus is a rephrasing of sentence 2.
A "man and a woman playing" is the same as saying "a man and a woman are playing".
There is a group of people because there is a group of people.