The user can add an Assistant bu themselves, as Administrator you can add an Assistant for the users as well.
You can add the 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 right that the user has:
If you would like to add the 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.