Discourse allows you to invite people by email address, which is IMO a nice feature, especially early in the life of a forum; that doesn’t seem to be working here. I wonder if it could be enabled?
I seem to recall email invites having some complicated relationship with the auth methods; they are, or were at some point, incompatible with SSO. As I understand we are not using that, though.
(I can also imagine social reasons why it would be hard - it can be abused, and it can annoy users who have multiple email addresses so Discourse thinks they are not registered yet even though they are. But at least early on the benefits outweigh those drawbacks, IMO.)