At first, the app 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 Confluence Administration → Manage Apps → Switcher – Easy impersonation):
Here you can define the groups of users who should be able to impersonate as well as the groups of users to turn into.
Leave the setting empty to grant the relevant permissions to all Confluence users.
The Audit Log tab contains the information of Switcher functionality being used by Confluence users.