IP Multicast

Hey everyone, I've been reading through a few tutorials on using multicast, and I've just got a couple quick questions:

1) When you're receiving, after getting membership to the group, do you call recvfrom() with the address of the group, or with the address of the sender?

2) Are most routers multicast-capable nowadays? And if the router you're plugged into isn't, then will multicast still work (just slower :confused: ), or do you need to install some sort of software or driver on the computer or something?