After a family funeral recently, we all gathered to eat and talk, but the family was split up into three rooms and three distinct factions. (There had been some feudin'. Don't ask.)
My first suggestion works even if you are shy, because it is a one-on-one thing. I picked out a big, tough-looking cousin, introduced myself and then simply led him around asking him to introduce me when I didn't know someone, and vice versa. He played along and it really broke the ice in all three rooms. Another idea is to ask someone to take your picture and hand them your own camera to do it. I don't know why, but it works, and ohers will quickly join in with their cameras.
You can also ask older family members to explain how many kids or spouses So-and-So had, or some such question, and all of a sudden everybody's an expert. And they're talking to each other.
P.S. Thanks for your reply last week about the "Confederate" reunion. I posted a thanks to all in the main forum.