Suppose there’s this site I really like, and I like the fediverse, and I think it would be awesome if they joined the fediverse, to benefit everyone involved. Is it like how it was in the Zorro movie where they vote California into the US, Zorro swipes all the voting boxes with a Z, and everyone cheers, or is it not possible for just any random schmoe site to join?
The software which runs that site needs to be compatible with the ActivityPub protocol. Most of them aren’t and it is very difficult and time consuming to add ActivityPub to random software which hasn’t been designer for it from the start.