Difficult one, I suppose you will have to hope someone will be reading this who was a hotel guest at the time of the episodes was being filmed. I have stayed there but of course knew it had been filmed there so had no reaction of surprise which I take it that you are after.
Only a handful of episodes were actually filmed on location at the Hotel Portmeirion (Arrival, Free For All, Checkmate, Dance of the Dead, Many Happy Returns and Hammer into Anvil). A couple of other episodes saw minor filming at Portmeirion (minus McGoohan), but all other Village scenes were accomplished via mockups at the MGM Borehamwood Studios in London, as well as via reuse of generic Portmeirion footage.
The hotel was open for business in the 1920s and was used in other programmes before The Prisoner so it was not as if no filming had ever been done there before. I would strongly recommend you have a look at the Portmeirion website ... there are numerous including a virtual tour which is fascinating. Following the links may put you in touch with people who may have been present at the time of the filming and will be able to give you first-hand accounts. Good Luck.