Ref T7152. This builds the core of email invites and implements all the hard logic for them, covering it with a pile of tests.
There's no UI to create these yet, so users can't actually get invites (and administrators can't send them).
This stuff is a complicated mess because there are so many interactions between accounts, email addresses, email verification, email primary-ness, and user verification. However, I think I got it right and got test coverage everwhere.
The degree to which this is exception-driven is a little icky, but I think it's a reasonable way to get the testability we want while still making it hard for callers to get the flow wrong. In particular, I expect there to be at least two callers (one invite flow in the upstream, and one derived invite flow in Instances) so I believe there is merit in burying as much of this logic inside the Engine as is reasonably possible.