Don't be confused by the verb "eingeladen"
They are focusing on the fact that this many people were present at the opening ceremony.
It could have been "waren anwesend(present)" instead, but "eingeladen" simply sounds more welcoming and humane, and I really doubt that most of these people actually got an invitation of any sort.
If they wanted to continue the story with the efforts it took to invited this many people and/or how these people received the invitaion etc, then "wurden eingeladen" would be the appropriate expression though. But the context is saying that it isn't the case here.