Nice to meet you.

Say this when you meet someone for the first time:

Nice to meet you.

For a little more formality, you can add "It's":

It's nice nice to meet you.

If you've met someone before and you see them again, don't say "Nice to meet you" again; they might think that you don't remember them. In that case, say:

Nice to see you.

Welcome aboard.

"Welcome aboard" is something that you say when someone gets on a ship.

In business, we sometimes compare a company or a team to a ship. One example of this is saying "Welcome aboard" when someone joins a team. It means "Welcome to the team."