Be able to bring for free external users to SharePoint online is great even, if they don’t have a MySite/Skydrive/Personal newsfeed,etc and cannot use the Site Mailbox.
But here is a side effect.
Let’s take a look at the following scenario :
an invitation to connect to a SharePoint Online site has been sent to the users firstname.lastname@example.org.
The user receives the invitation in his inbox :
and clicks on the link; the normal procedure is that he uses email@example.com as a Microsoft account : the following window shows-up
However, the external user has a existing private live id (let’s call it firstname.lastname@example.org) and doesn’t want to use it; he didn’t signout from this live id and no window is open in the browser.
The user naively clicks on Microsoft account. What will happen is that his private e-mail address (email@example.com) will automatically accept the invitation.
if afterwards, the user connect his professional address firstname.lastname@example.org to a live id and wants to connect to the O365 web site, he will get the following error message:
So basically the invitation is sent to identity A, and identity B is authorized.
And this could be embarrassing if the user doesn’t want his private e-mail address to be known…
The helpdesks won’t love it…
So my advice is : ask the partners to sign-out from their Microsoft account by going to www.live.com :