John Henry is first & foremost a song…it has singing. Kick-off is usually done by whoever can start the song well (to be decided at the time) and single breaks (unless told otherwise) when given the nod from the vocalist or leader.
In my experience banjo usually starts it out (if there is one) to differing degrees of success.
I have heard far more sessions do a “round” of breaks between the vocal parts rather than multiple breaks by a single instrument on this song…but that’s just my experience.
All that being said, each jam has it’s own character and members. Things change and it’s good to stay on your toes, know what’s acceptable for the group, and above all Listen.