A lot of what I'll be talking about can be lumped under the heading of "etiquette". For many, that word has nasty connotations of formality, rigidity, stuffiness etc., but really, it has nothing to do with that at all. Etiquette is really all about being considerate and appreciative of other people, and conducting yourself in such a way that you are pleasant to be around.
When considered in the context of music, there are some very specific things that can and should be done to make your session enjoyable and to make you a welcome participant wherever you go. Sessions or Jams can be organized and more formal or impromptu and less formal. What I will say here applies in either case, but some special rules might apply in more formal or established venues. I would suggest that you familiarize yourself with any standing rules at the session you are attending before you make a nuisance of yourself. Let us examine some of the issues pertaining to playing with others.
Tuning is fundamental. Do not attempt to play with an instrument that is out of tune. This is especially tough for accordion players, since their instruments can't be easily tuned. If you play with an out of tune instrument it will annoy an awful lot of people and the music will not sound good, no matter how good you personally are as a player. Yes, sometimes other instruments can tune to you, but should they have to? Its one thing to just be out of concert pitch (A=440), but another altogether to be out of tune with yourself (intervals not correct) - this can rarely be adjusted to by other instruments.
Related to tuning, tremolo is a feature of accordions that can cause trouble. If you have a wet tremolo voicing on your box, it can really annoy some musicians, particularly violinists. A very wet tuning can cause your instrument to sound out of tune, even though it isn't, and it can cause confusion as to which is the actual pitch of the note. Is it your M reed or your M+ reed? On a wet box, the difference can be great enough that it causes problems. My suggestion is to use a box with the driest tuning you have, or on an instrument with stops, silence the tremolo reed.
Because the button accordion has a more limited choice of chords than some other instruments (guitars etc.) sometimes the basses we use are not the same as what other musicians would use. Its good to have an understanding of what the bass progression will be for the tune. If you can't do that progression, then just play without basses, or try to fill in with harmonius fundamentals and leave chords off in the problem areas. Another option is to explain what you normally do and respectfully see if the other musicians are averse to trying a progression you can handle. Sometimes its fun for them to try a bass line that is out of the ordinary for them. If you really aren't sure about the basses, just play them very lightly and percussively, and they shouldn't be much of a problem.
In my opinion, this is the single biggest problem in most sessions. The music should be kept at a slow, steady, deliberate speed. Some musicians see a session as a place to show off their virtuosity, and for many of this mindset, speed is impressive. They think that they will impress their fellow musicians by setting a blistering pace that most can't keep up with. Nothing could be further from the truth. Playing too fast is often a weakness of beginners who succumb to the rush of adrenaline and nerves when playing in front of people. It can also be an indicator that the player is a show-off and a braggart. What inexperienced players rarely realize is that playing slower can actually be more difficult, and more enjoyable - not to mention that it often results in better music. It is not fair to try to make other musicians drop out because they can't keep up, just so that you can puff up your ego.
Another very common pitfall which should be avoided is the "runaway freight train". This often occurs in sessions unintentionally and it can really be frustrating. What happens here is that one musician will hit a note just a split second too early, for whatever reason (often unfamiliarity with the tune, or nerves). Because this note comes during a moment of silence in the music, or because it stands out as a different pitch, other musicians hear this note more clearly than when everyone is playing in unison. All of sudden everyone in the session questions whether they are falling behing the tempo, and many immediately speed up slightly to get back on beat. Now you have part of the group at a slightly faster tempo than the rest. The music starts to sound a bit jumbled, and the least experienced players wonder whether its because they didn't speed up enough. To compensate, they speed up even more. By now, the experienced players have given up trying to put the brakes on, resign themselves to another runaway freight train and speed up to join the rest of the group. This whole series of events can happen several times during a tune, with the end result being a tune that is so fast that nobody is playing it well.
If you have a really stubborn group of experienced musicians in the group, they can sometimes succeed in bringing the tune back to a reasonable tempo. Other times, the tune will just self destruct because the difference between the two tempos just gets too great. In either case, its not an enjoyable experience for anybody. It is much easier to speed a tune up than to slow it down. One way to avoid it is, if you think you may be behind the tempo, don't speed up. Instead, play more quietly and listen (especially to the more experienced players) to find out where the tempo really is. Once you've figured it out, you can return to playing a normal volume. If you are behind the tempo, playing quietly will help avoid throwing others off, and it helps you to hear the other musicians.
There are many in the world of traditional music who feel that tunes ought to be repeated many, many times, not just 3 or 4 as is usually the case in sessions. There are several reasons for this. One is that beginning players are usually just catching on to the tune after the 3rd or 4th time through - then its over. By playing it through more times, the beginners can get comfortable and really make some progress. Also, there are variations and harmonies which develop as the music progresses and the musicians play off of each other. 3 times through is just not enough to let that fully develop. Finally, the contention is that traditional tunes which have survived through the centuries to the present day have done so because they have something special to offer (call it a groove or whatever). They have a sublime appeal which often only reveals itself after endless repetition and thorough examination (think of it as musical meditation).
The modern trend is to play medleys so that the musicians don't get bored with the tune. In the old days, musicians would play the same tune for 20, 30, even 40 minutes straight. In the process, they refined it and brought the music as close to perfection as is possible. This is part of what is sometimes referred to as "folk genius", whereby unschooled amateur musicians were able to compose tunes which have survived centuries and still appeal to us sophisticated, modern, urban, worldly, educated people. It happened through slow and laborious refinement. I have heard the proposal that a tune should not be played a certain number of times through, but for a set amount of time - say 10 or 20 minutes - however many repeats that works out to be is irrelevant. Rather than becoming bored with the music played for such a length of time, the musicians ought to focus on the tune, take it in, refine it, explore variations, harmonies, phrasing, dynamics, find out what makes it work, etc.
A side benefit is that it makes for a more relaxing session. Musicians can drop out and come back in without worrying about missing out on the tune. It defeats the urge to rush through tunes that aren't as personally appealing, and it practically forces the musicians to deal with the runaway freight train problem. You can't play for 20 minutes if you start accelerating from the start.
Unless you are leading the tune, or you are an experienced player trying to solidify the tempo, there is no reason for you to try to play any louder than anybody else in the room. You are not there to show off or to try to dominate the session. Accordions are loud - try to blend your sound with the rest of the musicians. If you are not 100% solid on the tune or the tempo, you absolutely shouldn't be trying to play loud - on the contrary, play softly until you are solid, so that you don't throw others off. Although your instrument may not sound loud to you, and you may in fact have trouble hearing yourself, be assured that the person sitting to your right thinks you are plenty loud. If you need to hear yourself, lean your ear in closer to the box, especially near the grill - don't play louder.
Here are some good tips from my friend Gert-Jan in Amsterdam, who is much more experienced with sessions than I am, "When you can't hear yourself playing, it's no problem. When you hear yourself you are either playing too loud or just playing something different.
Sometimes when playing together and it tends to be messy I call for a break and ask for a new start where we all will listen to the other's playing. From that moment on the playing improves tremendously."