Just adding a note here to say that the issue seems to be that when invites are sent from a site that is using DiscourseConnect for authentication, users are logged into Discourse through the SSO provider site when they click the invite’s “Continue” button:
This means that their Discourse account is not actually created until they go through the authentication process on the SSO provider site. This is the expected behaviour. When DiscourseConnect is enabled users can only login to Discourse through the SSO provider site.
I’m not sure what could be done to make the process more clear. If the invited user doesn’t have an existing account on the SSO provider site, the process could be quite confusing.