As a French person, I only know Hugh Laurie from House MD or Stuart Little, which are both subsequent to the Friends episode. Can someone tell me why in this episode, Hugh Laurie is considered a Guest Star more than a simple background actor?
Hugh Laurie has been well known in the UK since the 1980s, appearing in such popular TV shows as:
And parts in the films:
Before 1998 when these episodes were made.
As these episodes were set in the UK a well known (and quite loved) British actor is likely to be credited as a guest rather than just an extra.
The general principle is that a "guest star" is someone who has been invited to undertake a role in the film or TV show. They aren't a star because of their major role in the production, they're a guest who happens to be a star.
Where they also have a large role, they might be credited as "Guest Starring".
Since it's highly unlikely Hugh Laurie (or Fergie) would have been expected to audition, that makes them guests.