As this group isn't listed in any directory, outsiders wouldn't even know it existed and even ex-members who have left would presume it was still locked up (if the group is even in their conscious memories still) so are unlikely to be probing to see if they can get into it. The only thing is that crawler search engines such as Google may locate the group and list it, although that would normally take 3-5 days to occur, and even though Google caches what it finds, my experience is that they don't cache the contents of MSN groups, but merely the homepage. Hopefully, by the time Google discovers the group, migration would have occured and you could slam the door shut again.