I dont know about 'jams' but Irish sessions, are not necessarily open to anyone who comes, whether in a public space or not. It really depends on the hosts, and the level of the session etc. Most people will ask if they can join in, and most are welcome, but if someone comes with djembe

We have never sent anyone away, but those who come, and insist on loudly playing the wrong chords or banging away at a bodhran regardless of the beat will usually be told discreetly to keep it quiet, or to bone up on playing chords etc. as they are in fact ruining the session for the other players. Weve also asked the percussion, guitars etc. to sit on the outskirts and leave room for the melody instruments.

There is another session at the same bar on another night, which is more of a beginner session, so we can direct beginners there.

We also have a French Canadian session, in which percussion is strictly limited, no bodhrans (certainly not part of the French Canadian tradition).

its all common sense anyway, and theres a continuum between having anyone to play anything.. and having an enjoyable experience
(one regular bodhran player, started handing out spoons and noisemakers to people at nearby tables, well you can guess this didnt add to the musical experience.)