Our local Folk and Acoustic Club (Vernon Arms, Spondon, Tuesday nights) simplifies things by not calling it a jam night nor an open mic night.
Anyone who shows up with suitable material gets two songs in their turn. Ability is not usually an issue. It is well populated each week. The host will usually kick off with a couple of songs that he accompanies with guitar. If it's a quiet night there are almost always enough players to go around a second time. Then, a few times a year, a folk act will be booked as guests. That's quite a good night too.
Isn't it funny that so many find jams and open mics more cliquey than a club night that charges a pound to non-members if you're performing and two pounds if you just want to listen?