When someone joins a project or a team, you can say that they are "on board".

A: We'd really love for you to join us.

B: Who else is on board?

A: Well, Takahashi, for one.

You can also use the phrase "on board" to talk about whether someone agrees with an idea:

If you're considering borrowing money, I'm not on board with that.

The phrase "on board" originally mean that someone was on a sailing ship.