The session I went to for years started as open mic night and morphed into unholy jam night once everyone got used to me. It stayed billed as open mic while everyone referred to it as jam night.
I would always ask new folk if they wanted the bass. Some should have said no, the odd ones who didn't know their own material well enough, the ones who couldn't keep time.
Imo all open mics should be so lucky as to have a bassist capable of sitting in with randoms grace them with their presence.