Adding Client To Business Groups

[RMortis]

I have a client with 20ish locations. All owned by the client but in business under different names. We manage all this for them, no problem, in our agency account.

The new head of operations wants corporate access to all these, which makes perfect sense.

What is the easiest way to grant access to 20 individual business groups (some with 20-50 listings inside) to one new account?

Thanks!