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.
```
We consider X to be an authorative source for user email addresses.
There's no need for the user to verify it
```

here and in the other strategy files