who was the man who played piano that was at the johnny carson show that was sitting in the audiance and asked to come on stage

The man was David Tolley. He was an audience member on The Tonight Show Starring Johnny Carson who was invited on stage to play piano when the scheduled performer couldn’t appear.

What happened

Carson asked whether anyone in the audience could play piano, and Tolley volunteered. He then performed live on the show and impressed Carson and the audience.

Why people remember it

The moment stood out because it was spontaneous and turned an ordinary audience seat into a live TV performance. Tolley later became widely associated with that appearance.

TL;DR: It was David Tolley.