IPv6 public -unicast- addresses have the prefix 2000::/3 (so far). The allocation for multicast includes accommodation for link-local, variously local scoped and global addressing (as per RFC3307). It's the same basic idea as IPv4 multicast, where a chunk of the 224/4 space is set aside for GLOP addresses and such.
Check out RFC3306 as well, I think it probably more directly answers your question.
This specification defines an extension to the multicast addressing
architecture of the IP Version 6 protocol. The extension presented
in this document allows for unicast-prefix-based allocation of
multicast addresses. By delegating multicast addresses at the same
time as unicast prefixes, network operators will be able to identify
their multicast addresses without needing to run an inter-domain
So the idea is that if you have a globally routed /64 that it can be included in the overall group ID to give you something that can be routed globally. In other words, if you already have a v6 prefix and a suitably enabled carrier then you're already set.