I just ran into difficulties with invites. I created a category where I only want people with level 2 permissions to be able to reply to posts and level 3 permissions to create posts. I then created a topic and then invited someone who is not already a user on my discourse to ask them to reply to a topic I created in the category, and they are not able to reply because they only have basic permission.
This seems to be a bug to me, since the email received explicitly states “You were invited by a trusted user, so you’ll be able to post a reply immediately, without needing to log in.” It seems that is only true for invites to topics in categories where everyone can read/reply.
Cheers,
Tobias