Q:

If you don't have 200 rep anywhere when you join a new site, is the Association Bonus added later?

When a user with >200 reputation on a Stack Exchange site joins another Stack Exchange site, they get a reputation boost, but is this boost only when the user first joins the new site?
That is, should I hold off on joining sites until I have 200 reputation on at least one stack exchange site, or when I cross 200 will all the sites I have joined get the boost?

A:

when I cross 200 will all the sites I have joined get the boost?

Yes. When an account goes over 200 on one site, it will get a 100 reputation bonus on all sites associated to the same account.
See How does "Reputation" work? for details.

