BuddyPress.org

@blogminded @tripvips
That would work fine unless the site admin was using a plugin like:
https://wordpress.org/plugins/private-community-for-bp-lite/
Then that URL would be useless and they would just get redirected but maybe this person is not using a plugin that restricts logged out visitors from the site so your idea may work for them.

@tripvips
Sorry but there is nothing like that out there that I have seen. You would have to develop this functionality yourself. It would be a little complicated as well!