Well, essentially, you don't. A sub-article to the above one,
IP Multicast http://en.wikipedia.org/wiki/IP_multicast, discusses the how's and why's. Kind of bear in mind that you're more or less just a passive receiver of multicast data; multicasting is a method of efficient
transmission, not reception (as pretty much explained in this article).
So, give this article a good read and take a look at the sub- sub-article
Internet Group Management Protocol http://en.wikipedia.org/wiki/Interne...ement_Protocol referenced in the
IP Multicast article above. That may be helpful.
Keep in mind that this is pretty complicated stuff and it's not all that easy to get your arms around -- it's fiddling with IPv4 and IPv6, routers, protocols and a whole specialized world that most of us never see. The Wikipedia article in the first post is a good place start, being sure to follow the links to sub articles.
Hope this helps some.