This article explains how to set up shadowing for custom administrators.
While logged on to the Access Management Console as a full farm administrator, create a custom administrator account with the permissions as outlined in CTX109578 - Shadow Button is Unavailable for Custom Administrators.
Create a policy in the management console to allow shadowing. Under User Workspace > Shadowing, enable the configuration and choose Allow Shadowing with the desired options. Then configure the users and groups that require the ability to shadow underPermissions.
Apply this policy to the shadowees (the groups over which you will shadow). Shadowing is enabled for these users within the Access Management Console and using the Shadow Taskbar.