How to isolate new OUs/domains from rest of pre-existing Exchange/AD OU's/GAL?

We have a pre-existing Exchange 2013 with 40 domains and 550 users on it. We just started a new company/domain that we want to put on Exchange but we DO NOT want any of those users to see ANY of our pre-existing OUs or users, including in the GAL. We also don't want any of our existing users to be able to see any users in our new 'company'/OU.

I've read about multi-tenant setup. However, we already have a pre-existing Exchange environment with 40 domains and over 550 users. Is there a way to set it up as a multi-tenant w/o having to redo everything?

To clarify... we have A, B, C, D, E, F on pre-existing Exchange 2013.
We want to add G and H w/o A-F seeing any evidence of the existence of G and H.
And we don't want G and H seeing any evidence of the existence of A - F.

The least impactful solution that will accomplish this is preferred, if there are options. The existing Exchange 2013 environment is working very well and I don't want to screw things up unnecessarily.
FrankU09Asked:
Who is Participating?

[Product update] Infrastructure Analysis Tool is now available with Business Accounts.Learn More

x
I wear a lot of hats...

"The solutions and answers provided on Experts Exchange have been extremely helpful to me over the last few years. I wear a lot of hats - Developer, Database Administrator, Help Desk, etc., so I know a lot of things but not a lot about one thing. Experts Exchange gives me answers from people who do know a lot about one thing, in a easy to use platform." -Todd S.

AmitIT ArchitectCommented:
GAL Segregation is one way to do it. How is your current environment setup? As you already have 40 domains. Are they able to see?
FrankU09Author Commented:
Yes. All users can see all other users currently. Segregating GAL sounds like the plan. How is this accomplished?

Experts Exchange Solution brought to you by

Your issues matter to us.

Facing a tech roadblock? Get the help and guidance you need from experienced professionals who care. Ask your question anytime, anywhere, with no hassle.

Start your 7-day free trial
It's more than this solution.Get answers and train to solve all your tech problems - anytime, anywhere.Try it for free Edge Out The Competitionfor your dream job with proven skills and certifications.Get started today Stand Outas the employee with proven skills.Start learning today for free Move Your Career Forwardwith certification training in the latest technologies.Start your trial today
Exchange

From novice to tech pro — start learning today.