Es ist der Unterschied zwischen Zustandspassiv (a) und Vorgangspassiv (b). (a) "Es waren... eingeladen" = They were invited (you just know that they received the invitation) (b) "Es wurden... eingeladen" = They were being invited (you were either present while the invitations were being discussed/sent or at least you are talking about the point in time when they were being discussed/sent)

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.

Schwierig zu erklären. "wurden eingeladen" ist auch möglich, grammatikalisch. They are both possible, grammatically speaking. Difficult to explain. Zur Eröffnung waren mehr als 2.000 Menschen eingeladen It focuses more on the 2000 people that have been invited. Zur Eröffnung wurden mehr als 2.000 Menschen eingeladen The focus here is on the actual act of inviting them. German is my first language (albeit a bit neglected having lived half my life elsewhere). I use it naturally, without really knowing the rules. It's the best explanation I can think of. Perhaps someone has a textbook answer. I'll keep an eye on the threat.