Yip, only authenticated users can add carpools _or_ send messages.  Otherwise they can just view details (like telephone numbers).  Ultimately they should get a link (or something) to signing up on the website.  Well, ideally they could do that through the app, but that's a future thing.

When a user sends a message to another user, their email address is passed through (which is slightly naughty, but still) then the two people can email offline.

So yip, pretty locked down unless you're logged in.