How are the people who don't include you in conversations? Are they still polite, or are they flat-out jerks? If jerks, there's something to keep in mind: jerks can stand out in our minds much more strongly than those who are polite. I was working with a couple of rude people once, and they completely ignored everything I said with a smile on their faces. They were enjoying excluding me, even though I continued to be polite and attempted to be helpful. They were also shallow and vain... I also had to deal with a large group of people who, unluckily for me, were also very shallow, vain, and competitive people. There was one girl in the group who seemed oblivious to this and was still very polite and responsive to me, and I regret avoiding her like the rest. I guess I didn't want to make her look "bad" in front of the vain group of people, so maybe what I did was the right thing. Just one person being kind to me, though, in that situation, made all the difference. Rude people can make normal, unimportant things like conversation so dramatic.

If they're polite, then I'd definitely go with what s4ndm4n2006 is saying. Maybe try interjecting once and a while with something they relate to? Or simply asking about what they're talking about, even if you don't know the subject.

I also avoid large groups of people. I'm also usually ignored... I think it may be because the average person in a large group subconsciously chooses a "leader" or "leaders" of the group, and follow their "direction" when speaking and listening. I think if you're ignored by the leader, seem timid, or just someone the majority of people don't know or care for, then you more often than not get ignored. I consider it a herd mentality. I'd only enjoy being in a large group of people I respect.