I left the band I was in back in '09. Have kept in touch with my co-singer; she performs informally with her piano-playing partner. She said "we're doing Billy Joel's 'Piano Man' next month. Will you do the harmonica part for us?".......... I said "yes... of course..." :$
It's not a dificult part.. can get all the right notes in the right order. But hard to make it flow; hard to make it sound musical. More practice needed.
I did the same several? years ago when I met with a couple old band members ( that band hadn't existed for more than 20 years! ). I was as rusty as hell but we had a good blast. Don't worry, you'll enjoy it.