Permission Question

[ephigenie]

Have you been taken a look at the demo ?

Because the functionality you're asking for is already integrated...
You can change into every user's interface from within the admin account.

So you can look into reseller accounts and client (domain - user ) accounts.
Your customers may edit their settings on their own with their own login and you may help them without knowing their password, because you can switch into its account through the panel.