How do I give a managed user delegate access to another managed user's account?

To give one of your managed users delegate access into another managed user's account, you must first be an administrator with subscription rights.
 
First, select Options in your left panel bar and select the Manage Delegates tab. Once in the Manage Delegates tab, select Add Delegate.

You will be prompted with a window to input a Delegate and a Delegated Member. A delegate is the impersonator as the delegated member is the impersonated. Therefore, if Jenny Wilson is set as the delegate, and Jared Davis is the delegated member, Jenny Wilson can impersonate Jared Davis. Once you have selected your delegates, select Add Delegate.

Jenny Wilson can then impersonate Jared Davis from her account by selecting the User Menu on the top right corner of the screen, and selecting Jared's panel underneath Switch user.