How do I add an Assistant for one of the users?

Can I, as Administrator, add an Assisant for one of the users?

Any user can add an Assistant for themselves, but Administrators can also add Assistants for other users as well.

You can add an Assistant by going to users via the gear. Click on the eye symbol next to the user and add an Assistant:

You can add an Assistant for the rights which the selected user has:

If you would like to add an Assistant for a specific period of time, you can select an expiration date. 

The assistant can switch between accounts by clicking on the drop down menu under the logo. For more information, read the following article: How to change between accounts or environments.