One of the coolest features unique to SharePoint Online is the ability to share sites with external users.  I’ve noticed the functionality was there throughout the beta but I believe I hadn’t actually been able to get it to work until GA.  Even then, it took checking a number of settings before I could actually get it to work.  You can share content with external users by using the new Share Site menu item in the Site Actions menu.  When you start with a new out-of-the-cloud site collection, you will get a screen that looks like this.

Notice the key text on the screen:

Invitations to users outside your organization are currently disabled.

To enable this, there are a few things to check.  First go to Tenant administration.  Your URL is usually something like https://mydomain-admin.sharepoint.com.  Go to the Manage Site Collections page and click Settings –> Manage External Users.  Make sure you don’t have any site collections selected otherwise Settings will be disabled.  Make sure the Allow radio button is selected.  It is more than likely set to this by default.

Once you have confirmed this setting, go back to the root site of your site collection and go to Site Settings and then Site Collection Features.  Here you will want to enable the feature External User Invitations.

Now, if you go back to the Share Site menu in Site Actions, you will now be able to enter E-mail addresses and the window will no longer have the warning about invitations being disabled.

I noticed it usually takes a few minutes for the E-mail to show up so be patient.  When the E-mail arrives, you will see something like this giving the user a link to click on to join.

Clicking on the link takes the user to a page prompting them to login.  They must use a Windows Live or Microsoft Online Services ID to login.

One thing to note is that the E-mail address associated with the Windows Live Id does not have to be the same as the E-mail you sent the user.

Don’t bother E-mailing me at that address, I never check it.   As an administrator, you can now also manage this user in the Site Permissions page like any other user.

At this point, it seems like the user can do whatever they need to on the site with the users you provided.  I’m not really sure how the licensing works in this scenario.  These users don’t show up in the licensing page so I guess you don’t have to license them.  Maybe an SPO licensing expert will chime in.  :)  There may also be additional limitations that I have yet to discover.  If you find some be sure and share them with us.