Honestly, I'd hope if lichess could add a feature to allow competition organisers to add a certain list of usernames who have registered for online tournaments set on this platform. One reason is because this circumvents the password to the tournaments from being shared out in groups eg. or to people who have not registered for the online tournament.
this feature has already existed for years.
as luck would have it, just recently someone inquired about it:
https://lichess.org/forum/lichess-feedback/fixed-line-up-in-lichess