A Microsoft MVP on Windows Live Messenger has confirmed in a forum that this is a known Windows Messenger bug and will be resolved in a later release. This is not a security flaw and so does not warrant a high enough priority.
Instead of waiting for a new release, the below method can be attempted to remove the annoying invitation alert.
- Close Windows Messenger (Make sure it is not running)
- Go to C:\Users\
AppData\Local\Microsoft\Windows Live Contacts. Note that it is required to have "hidden files and folders" shown to browse the above location.
- Remove everything in the aforementioned location
- Start Windows Messenger