This process outlined here is also applicable to assigning a device.

Start by creating a View which will the "container" for the device or interface that you want your customer to see.

Go to Edit Mnemonics and assign the mnemonic (interface) to the view that you created.

Find the mnemonic (interface) and click Edit. On the Edit Mnemonic pop-up, in the Views section, start typing in the view you created and click Add.

Select Manage Users and create a user to access the system.

In the User configuration add the View to the View Filter field and click Save.

The User you have just created can now only see what you assign to that View, the same process works for assigning a whole device.