If you’re not sure what Office 365 is, take a quick look.
To be able to share a Office 365 SharePoint site with external users:
1. Go to portal.microsoftonline.com -> Admin -> Manage SharePoint -> Manage Site Collections -> Settings -> Manage External Users -> Allow -> Save
2. Go to the site collection that will host the site to be shared -> Site Actions -> Site Settings -> Site Collection Administration -> Site collection features -> External user invitations -> Activate (if not already active)
3. Go to the site you want to share (this doesn’t have to be the root site of the site collection, can be a subsite) -> Site Settings -> Share Site. You can type what e-mail addresses you want, they do not have to be Live IDs or be on the hotmail.com, live.com etc. domains. A message confirming that the invitation has been sent will show up. Access the invitation link in the e-mail and you’ll be redirected to a login page giving the option to login with either a Live ID or a Microsoft Online Services ID. You can logon with any Windows Live ID that you might have, it doesn’t need to have the same e-mail address that the invitation was sent to.
You can even give an external user contribute rights to a site, which I think is a very interesting feature for collaboration with customers / partners.
I’m watching this ability to collaborate with others in Google Docs and Skydrive for a long time now, and I consider Microsoft’s approach more balanced from the security vs. usability point of view than Google’s, which enables editing without requiring a valid account.
Hope this helps.