Hello there,
If you want your existing security groups to be merged with the new security groups then you must create a nested group. You can add an existing Security Group to another Security group (also known as nested groups), creating a member group (subgroup) and a parent group. The member group inherits the attributes and properties of the parent group, saving you configuration time.
You can use Add-ADGroupMember to add a group to another group. Here's the syntax:
http://technet.microsoft.com/en-us/library/ee617210.aspx
Manage Azure Active Directory groups and group membership https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/how-to-manage-groups
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