Ask the new kid to teach you something. Nothing makes someone feel like they belong faster.

Belonging arrives the moment somebody needs something from you, not the moment they are polite to you. Being welcomed can leave a new person permanently in debt, while being asked to teach something puts them on level ground straight away. Everybody knows something the rest of the group does not.

Find out what the newest person is good at and ask them to show you. Ask in front of other people rather than privately. It takes two minutes and it changes their position in the group.