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Active Directory (AD) is a Microsoft brand for identity-related capabilities. In the on-premises world, Windows Server AD provides a set of identity capabilities and services, and is hugely popular (88% of Fortune 1000 and 95% of enterprises use AD). This topic includes all things Active Directory including DNS, Group Policy, DFS, troubleshooting, ADFS, and all other topics under the Microsoft AD and identity umbrella.
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you have a hybrid exchange deployment which only 25 mailbox migrated to Exchange online (congratulations on that!)
so to simply the use is to assign the licence to the right users.
Once you assign the licence, then the user will able to access the yammer services.
you don't really need their mailbox to be in office 365 to utilise any services (for example, skype for business, yammer, onedrive for business and etc)