You can do that by leaving them alone.

Don't call attention to them.

Don't ask them to talk in meetings, or to comment, or to share their opinions. They'll decide for themselves if they want to do any of those things.

Don't call on them in class.

Don't ask for volunteers and then pick people (like them) who didn't volunteer.

Don't demand they speak in front of groups - unless that is really, really necessary.

And if you believe it is, find ways they can do that without forcing them into situations that will trigger their anxieties and fears.

Sometimes, written correspondence, email, and social media will be satisfactory alternatives.

But if that is not enough, and your organization needs them to be more personally outgoing, help them to develop that ability, but in ways that are comfortable for them.

Don't force them to do things the way you do. That just won't work.