I so love this feature - it is super useful and we’ll be utilising it a lot over the next two years.
I’ve noticed a problem for my users: if a user who is already signed in to Discourse on that browser attempts to use the link, they get this message:
This means that my group owner (who has been sending the link about) has to instead add them to the group using the Group page. No biggy, but it means unnecessary confusion and emails flying about just when we are trying to make a good 1st impression.
What I suggest is that if an existing user attempts to use the link, then they are automatically added to any groups specified in the invite and counted as a successful invite.