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.
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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?

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