Looks like a one-to-many (one user can have many bookings). You'll want a User table, a Booking table and an intersect table (UserBooking) for 'many-to-many', unless this is the funky Dataverse many-to-many (which just hides the intersect table and causes trouble).
You want to find all users who do not have a booking (UserID exists in User table but not in Bookings table). How you do this will depend on your datasource. If you are using SQL you can do this in the DB by creating a View:
Select
UserID
From
Users
Where
UserID Not In (Select UserID from Bookings)
For SharePoint things are trickier as you cannot process this on the 'DB'. You can write a similar filter to the SQL above in PowerFx:
Filter(Users, Not(UserID in ShowColumns(Bookings,"UserID")))
...but it will not be delegable and will first become slow and later fail to return correct results as the data in the lists grows and then exceeds the delegable limits.
You can try the same in Dataverse as 'Not In' might be delegable now (Dataverse appears to be the primary focus for MS and so gets more new features/capabilities than SQL or SharePoint), but if not, you will have the same problem as with SharePoint.
