you need to create a .NET passport.
It's free, and you can register if you click here
Once you have the passport you can sign into Messenger. You can also add your friends to your contact list - but they'll need passports too. MS Messenger will automatically send them an email invitation if they don't have a .NET passport yet. You can add your own message to the email.
Altrnatively, if you have a friend who is already signed up you can ask him/her to add you to his/her contact list - then you'll receive an invitation.
That's what you saw on Google.