It's also easy to forget that the mindset of everyone doing everything in a group and everyone being included might not suit everyone.
I had the exact same thoughts as you, wondering how it would go for my kids since they weren't in the center of events and I'm an outgoing introvert myself who didn't thrive in the spotlight.
But... as they get older, it often becomes a bit easier to find smaller groups to play in, more smaller activities where they can find what suits their personalities better rather than one big outing that's supposed to suit everyone. I actually think it's been easier since they started school, though the opposite can happen and challenges can arise instead, but I guess we've been lucky. What I'm trying to say, at least, is that everyone has their own journey and doesn't fit into the same mold.
As someone mentioned, I also believe there are small steps parents can take to build their confidence and cheer them on.