This article needs updating. Here are the instructions we were given

To assume an agentidentity:use the URL: [yoursubdomain].com/users

You'll be taken to a list of users
Click on Agents below the search bar
Find the agent you want to assume the identity and hover your mouse close to the edit button
Click on the Assume button
You will be taken in the Guide page, click on the products button upper right (four squares icon)
Go to Zendesk Support and test tickets, etc
To leave the assuming mode, go again to the URL: [yoursubdomain].com/users
There will be a yellow message on the top of the page with an option to end assumption mode