In one of my projects we are thinking about using Office 365 for an extranet scenario. A good story for another blog post. Depending on the plan we use, up to 10.000 external users could be invited to collaborate in a SharePoint site. See the service description for further details.
One question we had was, how can we determine, how many external users are invited to any of the sites. The answer to this question is very simple. With the SharePoint Online Management Shell we can use the cmdlet Get-SPOExternalUser. The result shows all external users currently in use in any of the sites.
Just for your information, there is also a cmdlet to remove an external user: Remove-SPOExternalUser.