Two different things. A subnet mask just tells your computer what IP addresses are local and what aren't. (Why would you use your cell phone to call your friend, if he is standing in the room next to you? You wouldn't.)
Multicast refers to the fact that you are sending data to multiple IPs rather than a single IP. (How many people are you telling at the same time?) You might end up using your cell phone (router), to contact those additional people so you can say what you need/want to say only 1 time.
I hope this sheds a little light on the subject.