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As I understand currently you are syncing only one OU which is in Sync scope and you have one group which is part of this OU.
Now you want to sync all users part of this OU to Azure.
Once this configuration is done, users who are part of this OU will get synced to Azure AD. And if you also have the groups which are part of this OU, even groups will get synced and by default these users whoever is part of these groups will become member of same groups in Azure AD.
to sync both groups and there members to Azure, you need to put both users and groups in the sync scoped OU.
If you do not have users part of synced scoped OU and you only have groups part of synced scoped OU, then only the groups will get synced and there will not be any members in it in Azure.
If you do not have groups part of synced scoped OU and you only have users part of synced scoped OU, then only users will get synced to Azure AD.
Let me know if you have any further questions.
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