Inviting guests

This is the initial stab at inviting guests. It's **NOT** ready for in-depth review yet. I would however appreciate any feedback you can throw at me from a high-level POV.

It builds on a couple of other PRs, if you have any remarks about those, please file'em there.
Is this something we're going to allow for? Aren't we going to make entering a valid email address compulsory?