Since 7.0, a reseller's staff member cannot log in to Control Panel as a customer if they do not have an 'Own Users' privilege set to 'Manage' level or higher. The UI error message appears:
"You do not have the necessary permissions. Please contact your account administrator for assistance."
The cause is a software related issue known as #APS-38409 "View mode of "Own Users" privilege does not allow to login as customer".
As a workaround, set 'Own Users' privilege to a 'Manage' or 'Admin' level for the reseller's staff member under System > Settings > Security > Roles > Choose a role > "Privileges" tab > "Edit" button.
Contact your Technical Account Manager or Pooled Technical Associate Team in order to find out the status of #APS-38409.