Most of the music played is bluegrass, from the discographies of legends like Bill Monroe, The Carter Family, and more. More recent songs from the likes of Allison Kraus, Lonesome River Band, Ricky Scaggs, Tony Rice, and other can be found. ‘Old Time’, Jazz/Western Swing and Celtic songs are often brought in, along with older Country songs.
There are no true rules on what can or can’t be played – we often joke about being arrested by the “bluegrass police” when the odd Beatles or Rock songs come out, but it is recommended to get to know the other players a bit before you call one of those songs in a larger jam session because not everyone enjoys those.
Instrumentation is all acoustic – the only exception is small amplifiers kept at a tasteful volume for electric basses, because not everyone has an upright bass or a vehicle large enough to haul one. Banjos, dobros, fiddles, mandolins, guitars and basses are the most common instruments, but other instruments are welcome – again, with the caveat of being considerate of other players by doing your best to stay in tune, in tempo, and at a reasonable volume so everyone can hear each other. Some instruments can naturally be quite loud and accidentally drown out quieter instruments and singers.
Generally the players sit in a circle, to best hear and see each other. Players take turns calling songs, most often going around the circle in one direction or another to give everyone at least the opportunity to chose a song. Most of the time if you call a song you’ll start playing it, and then people pass the lead around the circle, confirming by talking or eye contact that the person wants to take a lead. Improvisation is encouraged and supported – and players of all ages and skill levels are welcome. If you feel lost, don’t hesitate to reach out to others – mentorship and teaching each other is another strong tradition here. If there is enough interest, people have been known to hold beginner jams to teach the basics of this particular style of jamming, or even classes on different music topics.
Here are some websites with tips, and their version of the ‘jam commandments’ – they are definitely recommended reading. https://drbanjo.com/bluegrass-jamming-basics/
https://www.ocregister.com/2008/01/06/10-commandments-of-bluegrass-etiquette/
https://bluegrasstoday.com/the-ten-commandments-of-jamming/