I was asked if you can use SharePoint Online in an extranet or internet site capacity. The answer is no for internet sites but I have found out that you can invite up to 50 external contacts to your SharePoint Online site free of charge. Here’s how.

End-to-end steps to enable and external sharing with SharePoint Online:

The customer’s SharePoint Online Administrator turns external sharing ON for their whole tenancy. This merely enables the feature, but does not yet expose anything externally – merely lights up the capability at the site collection level
Set up a new site collection, or convert an existing site
Set external sharing to “Allow” at the site collection level. Note: you can always reset back to “Deny” if you no longer wish for the selected site collection to be shared externally.
Site Collection owner can now invite external trusted partners/customers by going to Site Actions “Share this Site”
External users get invited by site collection owners
- They are invited with any email address
- A Hotmail account is required to start login. End user will have the choice to use an existing Hotmail account, or be prompted to create one.
- The end-user would use their user@hotmail.com as their O365 login user name. We simply create a Personal User Identification (PUID) in our system associated with this original Hotmail account and associate it to the site collection they were invited to.
End-user is now known at the SharePoint Online Directory Service (SPO-DS) within the site collection they were invited to

Note: steps 5b-5c are not required if the invitee is already an Office 365 customer. They both have Microsoft Online IDs which will be used to authenticate them, and thus they just need to be officially invited into a site collection across tenancies …

Also please note I haven’t actually tested this myself!