EDIT: Did a little more research and found this on the .Mac Web site
Q: Can PC users belong to .Mac groups?
A: Yes. PC users can create a .Mac ID (and get a free trial membership) just the way Mac users can. The .Mac ID is theirs to keep and use for .Mac groups, AIM and iChat chats, the iTunes Music Store, and the Apple Store, even if they don't join .Mac when their trial memberships expire. PC users will be able to view and create group web pages, send and receive messages, and view the group calendar. Some PC users with software that has support for webcal protocol or webcal://URL can also subscribe to the group calendar.
Seems to address the issue I listed below.
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Some nice new features and the increased storage is overdue, but nice to see it moving forward.
One gripe, Groups would be very cool, but you need a .mac account to join a group you were invited to.
While that makes sense for apple, I have no use for the group thing because any group I would form would require many people without macs, let alone .mac accounts.
Come see my pictures online and share a calendar, it will cost you $100 a year. It's free on Yahoo.
I hope I am missing something, but when I was trying things out it looks like anybody who wants to participate in a group needs a .mac account