I am having the same problem as in this thread:

When I go to people.live.com and select Manage, Invite on Messenger, and enter someone's ID, I get the error "This page is temporarily unavailable" and its been "temporarily unavailable" for months now.

I don't even use messenger, but apparently I need to add messenger contacts if I want to allow people that I have shared documents on Skydrive with, to actually comment on those documents and photos (despite my requiring them to sign in with their Microsoft account they already have).  I think this design is ridiculous, and I don't want to change my security settings to allow anyone and everyone to comment, if I had publicly shared something.  Why do Messenger contacts have anything to do with Skydrive comments?  Especially when the people I share with already have a Microsoft account and are being required to sign in?  That's not enough for them to comment on what is showing under their SkyDrive -> Shared With Me section?

So, although I discovered this because of a SkyDrive issue with allowing my friends to comment, I've since discovered, although poor design, the problem is Messenger related, especially with the people.live.com website not allowing me to invite messenger contacts (in order to allow my SkyDrive friends to comment).