Call them anything you like, they won't answer back!
A guitarist.
a guitarist
Guitarist
A session musician; although the Beatles occasionally tried playing violins - who could resist trying out session musicians instruments - none of their efforts made it onto records.
A "harmonicanist" is what a person who plays the harmonica is called.
solo
I Call Your Name Ticket to Ride The Lead Solo on I've Just Seen a Face A Hard Days Night If I Needed Someone What Your Doing Eight Days a Week
Well Tommy Emmanual is an example. Even Elvis, he played the acoustic guitar too!
A bassist
a harmonist.
An organist
an "accordionist"