CSP partners come across a problem when they want to manage azure AD from the old portal, when they try to access manage.windowsazure.com they can not do this because CSP uses new ARM API.
However they can use aka.ms/accessaad link and signup to get access to manage azure AD with old portal.
Now you can manage users/groups/multifactor authentication etc for their customers.
Alternatively you can use other subscription like EA/MSDN (if you have access to any one)
and add your directory in this subscription and can still manage Azure AD.
If you have any query please leave comment.