Jamming Etiquette

Old time music jam etiquette isn’t that complicated, but there are definitely some unspoken rules, including

If a jam is billed as an “open jam”, then anyone can join in, regardless of how well they play or how long they’ve been playing or what instrument they play. In general, “open jams” will have a leader. Also, if there are more than 20 people in a jam, it’s pretty safe to assume it is an open jam!
All other jam sessions, you should wait until the musicians are between songs, then ASK if you can join in. The musicians may say no, and this can be for any number of reasons, such as that they have enough fiddles (or banjos or whatever) already, or they don’t care for hammer dulcimer, or that they are rehearsing, or they are longtime friends who haven’t seen each other in years and just want to be together in their own little group. If the musicians say no, don’t ask why — either hang out and listen (always okay!) or find another jam and try again.
Even if you know all the people who are playing in the jam, it’s STILL polite to ask if you can join. It's good practice to always ask, and sometimes people say no. Some folks feel that more than 2 of any instrument is too many.
Sometimes it will be hard to find a jam that you can join in which case the best thing is to find another musician (maybe someone you already are acquainted with) and ask them if they’d like to play a few tunes. In other words, start your own.
Instruments other than fiddle, banjo, guitar, mandolin are not always welcome in old-time jams. Also, louder instruments such as banjo-uke, or washtub bass, or spoons, may not be welcome. Another reason to ask first.
When you join a jam, if possible go into a corner and tune up beforehand. It can be distracting to sit next to someone trying to tune when you are trying to pick up a new tune on the fly. Having a clip-on tuner is a great boon in these situations!
If you are a beginner, play softly. Find the best player of your instrument and try to imitate how they move their right hand, that will help you get into the good rhythm, and even if you’re not getting all the notes of the melody, you’ll still be “on the train”. There are some exceptions, but usually, in old-time music, rhythm trumps melody.
Don’t Panic.