According to Mess.be, Windows Live Messenger (the next version of MSN Messenger) is now open to all – you don't need an invite to use it on your account. NO, RLY. Open to all

It seems like the Send Invite menu item has disappeared for beta testers, further confirming this. 

You can download the english version of Windows Live Messenger Beta from Microsoft.

Can't wait for a compatible version of Messenger Plus.