When I invite students to my class SkyDrive  group the response is "cannot be invited because cannot join any more  groups".   The students have assorted email accounts, but some like *** Email address is removed for privacy ***,jonas *** Email address is removed for privacy ***@hotmail.com are among those who created a hotmail account to be able to join the group and set up their own linked group to the class group.

These are only a few examples.