Give the rookie a real job on day one, even a tiny one. Nobody feels part of a team while holding nothing.

Belonging comes from contribution, not from welcome. A new person standing with empty hands has no way to be useful and no way to fail either, which sounds kind and feels like exclusion. Even a small assignment gives them a reason to be in the room and something to talk to people about.

When someone joins anything you are part of, hand them one genuine responsibility within the first hour. Make it real enough that its absence would be noticed. Then let them do it their own way unless it actually matters.