Microsoft started inviting a select group of beta testers to test the upcoming MSN Messenger beta. They can find the beta of MSN Messenger 8 at the Connect website using unique invite keys for each person. But it appears that testers will be able to invite their friends to participate with them.
"The MSN Beta Team would like to invite you to sign up for an upcoming MSN Messenger beta program. During this beta program we will be testing a new version of MSN Messenger.Source: NeoWin
We are looking for a group of testers willing to actively participate and provide useful feedback to our development teams. This will be a short beta program lasting only about 5 weeks".