I agree that joining a conversation in progress is a skill. I’m not terribly great at it. One thing I’ve tried is to listen for a while and see if I even want to join the conversation. Some conversations just aren’t interesting OR, I don’t have anything relevant to say. If I do like the conversation and if there is an opportunity to laugh or contribute something to what somebody else has just said I will do that. I think most people are probably ok with others joining the conversation so long as they don’t try to take over and change the subject. Just my thoughts but yes, I do understand that it’s tricky at times and sometimes people really don’t want to include others.