At first, the Switcher application should be added to your Jira. Read how to find, add and manage the Server apps (Atlassian documentation).
After installation click the Configure button (or find the app in the list of Teamlead apps in Jira Administration → Manage Apps → Teamlead → Switcher Settings) and specify:
The groups of users that should be able to work on behalf of other users (all users of the selected groups will be able to use the app functionality – check the User's Guide)
The groups of users who should be available to turn into. By leaving the field empty you allow all users to be available to turn into.
The Audit Log tab contains the information of Switcher functionality used by Jira users.