My internet has an upload limit of 1 Mb/s. If I am in a UDP multicast group with 10 people and I send something to the group at a rate of .1 Mb/s, will my upload rate be 0.1 Mb/s or 1.0 Mb/s? Assume all people are on the public internet.
Multicast is a special case of broadcast. It is a one-to-many mechanism. The sender sends a single packet and the network copies it everywhere it needs to go. To a switch or WiFi access-point, it's handled just like broadcast traffic, 'tho multicast filtering may apply. To a router that isn't configured for multicast routing (which is basically every router, ever), it's random broadcast noise.
For this to work, the various routers between you and your targets would have to be configured for multicast routing. This is unlikely. While it is, in theory, possible within a single ISP network, across the internet would be unmanageable. (not to mention immediately abused)
 This is why Uverse uses a dedicated AP for their wireless set-top boxes. Those APs run special software that handles the multicast video stream(s) like unicast. Wifi broadcasts go out at a basic rate (a minimum every associated client can handle) that's just far too slow.
The sender will not duplicate the content so your rate will remain .1. You will need to tunnel your multicast traffic across the Internet. Specifically, you may want to tunnel multicast over a GRE tunnel.