I've created a group in Microsoft SkyDrive, with a Group Calendar to share with my friends. In the past, I was able to send an email invitation to others. So users can check their mail, click on a button to join. Just the receivers will receive these invitation to join group in their Junk mail (this is unbelievable enough, Microsoft treated its own mails as "junk"!).
Recently, strange things happens, even in Junk box there's nothing found. The invitation to My Group is really sent, and if I try to send again, it told me that I'm still waiting for the user to reply...
After searching through so many settings at the user side, what I found is that Microsoft HAS CHANGED its way of handling invitations!
Solution: Ask the user to log in their Hotmail/Outlook account, go to People, Manage, View Invitations. There you go! REALLY, MICROSOFT IS UNBELIEVABLE!
Ask the user to log in their Hotmail/Outlook account, go to SkyDrive, Groups, View Group Invitation.