So, you say we name-drop an existing memrise user who isn’t already in the forums such as @Neil_LeFevre7a and should they by chance join at some later date then there will already be a notification for them - spooky circumstances for privacy nuts.
And if you post their email account in a public forum 
they will respond by registering your email address at every possible opportunity at every site they ever visit on the Wild World Web until you abandon it through sheer frustration. 
No. As I understand it, the “invite” function, will only operate in one of two ways.
If the invitee is already active on the new forum, you can invite them to a particular forum topic by using their Memrise username preceded by “@”. If they are not already active on the forum, the invitation will not reach them. If they join the forum at a later date, I don’t believe any message (“spooky” or otherwise) would be triggered as a result of an earlier ‘failed’ invitation.
Because the forum is a commercial off-the-shelf platform, I understand that it is possible to invite someone to a forum topic (who either is a Memrise user but not yet on the forum or not a Memrise user - at least that’s what the Discourse people say) using their email address in lieu of a username. The invitee’s email address would not be visible in any public area of the forum. And, no, people certainly shouldn’t post email addresses in public areas! I think the forum was designed that way to cater for those other websites using it which don’t have membership and usernames in the way that Memrise does.
Righto. I used the invite button. That’s pretty neat. I guess I’ll hear from him one way or the other 
I had no intention of posting his email address out there like that and having mine whenceforth broadcast to all and sundry. I’d have to be waaay more anonymous than this to get away with that 
Thanks for your help @alanh