Woodsie.
Unfortunately it is a fact of life that a heck of a lot of sessions are an enclosed clique. There are a number of reasons/explanations why this occurs.
Frequently it is because one or two dominating individuals just will not shut up and give anyone else a chance. They leap in with the next tune they want to play before anybody has chance to draw breath at the end of the previous set of tunes. They are not interested in listening to anything which an outsider might play, because it is not on their mental "playlist". In general I do not refer to these people as "Musicians" because to be a musician you need to listen and adjust your playing to what other musicians are doing. These people are instrument players (technicians if you like) who just play the notes, often a quickly as they physically can (plus missing out notes to make it even faster) to exclude anybody they see as "not fast enough to keep up" and therefore "inferior musicians" in their eyes. Essentially they are just selfish and are not interested in anybody except themself or their specific bunch of cronies.

In some cases a session "belongs to" an individual - it is "his/her" session, they started it and they set the agenda. They are there to play Irish/English/Obscure Catalonian stuff they nicked from the repertoire of some band you or I have never heard of....
Sometimes the leader is simply so carried away with their own repertoire that it will never occur to them that other people might have a different musical interest. You are not going to change this sort of session so a general rule might be "If it suits you , you join in. If it doesn't, you go elsewhere."

Often the session is dominated by "whoever plays the loudest" as they will start a new tune even after you have already started to play one, and just drown you out until you give up in disgust. I recall one occasion when I started playing Grandfather's Clock (Key G) in a session. A specific melodeon player joined in (In the key of D) and did not even notice that she was completely out of tune with what I was playing. Some people are just completely insensitive to anything except their own personal universe.

Sometimes the session is specifically set up with a musical agenda. For instance, every now and then Banjo Ray organises an Old-time American music session in Doncaster. I go along to that because I don't often get the chance to play Old-time with other players. On these sessions I don't want to play French tunes or Archaic English Dances, which I would be happy to do in a different location.

In most cases you will be included if you join in with what is already being played, but not if you try to change the agenda.

Some session leaders are open to new ideas, and if you get the chance to chat with any of the people attending, you may find that the session is not as exclusive as it might seem. Sometimes sessions get stuck in a rut, and are just waiting for one of the players to throw in something a bit different. You will not know unless you try.
In other cases it would be seen a someone "muscling in" and might be resented. If you have a chat (at the bar perhaps) with some of the players, you might find the situation is not as it appears to you.

Quack!
Geoff the Duck.