Thanks!
Understood, but then this was the admin’s option to make the invite link expire after a certain number of uses or after a certain date (in addition to limiting it to specific email addresses.) For us because we never wanted the invite link to expire the expire date was 2092 and the number of uses were 1000000 (and of course didn’t specify list of email addresses because we wanted the invite link to remain open to whomever actually wants to join the discussion topic in the private category)! However, now I can’t see how these options are going to be as useful when the link will expire anyway after the first use for each individual user.
Personally, I still believe that if this was enforced differently by adding an option during the creation of the invite link that’s similar to the constraints above (by default the invite link will not be reusable unless the Admin unchecks this option and once unchecked there is a warning about the security issue that the discourse team is concerned about.) It would have made everything so much easier from my perception. 
Thanks! This is exactly the approach we are currently taking; however, we need to update the guidelines with screenshots and at this moment we are still waiting for the fix that adds the ‘login’ button to the header: No 'sign in' option in the accept invitation page, only the signup form is shown
Thanks @martin!