Got the answer directly from the Rankade team:
You can add a new player during the faction step. Just type their email, and
then choose if you want them to be a 'member' or a 'guest'. Guest user
cannot access group data and is not listed in the rankings.
You can 'upgrade' a guest by reinserting them in another match and calling
him 'member'. Ghosts are users that are not linked to an email address.
They're managed by the group's admin.
A Guest user is not listed in the rankings, but he has his (hidden) ranking, indeed. So, yes, the guest "changes the ranks even if he isn't ranked himself" [it's a quote from the question], and you have to insert the actual order (ree calculations depend on all standings, so you need to correctly enter the full results, not just the winner).
We recommend the use of actual users, because some features (matchup
stats and more) are not available for guest and ghosts.