Two exchage servers and Office 365

I have two exchange servers in different companies and also in different AD forests. One of them is setup in hybrid mode for Office 365 with a few hundred senior people already on Office 365. I need to bring some users in the other exchange also to the same Office 365 so they can collaborate. How can I easily do this as they still need to retain access to everything they have access to now in their current forest.

The crude method would be provide an account in the domain that Office 365 runs in, this would be quick but also a pain both for the user and for admin. Hopefully someone has done this already. I'm dealing with a large volume of users so any migration would require considerable effort and resources, thanks.
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It very much depends on what's the expected end-result is. The high-level approach would be to provision and merge IDs from/to on-prem from/to cloud. Without downtime, that can be achieved only by utilizing AADConnect and Exchange Hybrid.

Overall this is quite an advanced topic, which will require a lot of planning and configuration.
Sid_FAuthor Commented:
Thanks I've seen the articles previously and been though them a tonne of times. I'm really looking for someone who has done this and can advise on the best route. One domain is obviously ideal but is to be avoided due to the enormity of the project.
My ideal goal is to have senior staff on office 365 using SSO and other staff on premise. A shared GAL with out an expensive GAL sync tool.
Jessica KCommented:
Hello Sid,

Go through the Microsoft's article as suggested by Ronin above. As well as, read the following blog carefully to resolve your query:

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Microsoft's recommendation is simplify the on-prem as much as possible, that probably means in your case to consolidate the domains and afterwards migrate the data. There could be quite a lot of issues around it, starting from available connectivity between the organization and ending with who's going to manage each forest.

That probably would be as far as it goes.

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Sid_FAuthor Commented:
It won't be me doing it however every one I speak to advises against consolidation due to the complexity time and risk but very few have done it! I am also not just consolidating one or two forests! I saw the multiple forests into one cloud identity as a good way around this but with exchange is now proving very challenging.

Is a resource forest an easier option? Move all mailboxes to resource exchange and create linked mailboxes for everyone then migrate users to O365. Again lots of technet but not much personal experience from people, thanks
Sid_FAuthor Commented:

Here is one article for forest migration so I'm still preferring the resource forest option. I'll open another question to confirm consolidation.
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