Child Domain Trust Relations

HI All,

  We have a HQ in US and Domain name is us.domain.com. They have few Child Domains.

  Parent Domain:  us.domain.com

  Child domains:  contract1.us.domain.com

                           contract2.us.domain.com

Now we just acquisition new business and domain name is newbusiness.com

We have create the forest trust between  US ( Outgoing and incoming trust proerties)

 Domain Name                              Trust Type                    Transitive 

  us.domain.com                               Forest                          Yes

  Contract2.us.domain.com                 External                      No

Now i need to give permission to contact1 and 2 Administrators to admin  newbusiness

But i cannot add those users to  newbusiness\domain admins ? 

As