To log in as a member:
Select the Manage > Members:
You can search for a member by name or email as well as filter your search by gender, and sort the search by name, last log in, username, birth date or date joined. Once you find the member you want log in as, click their name:
From the member details page, on the right sidebar click on the log-in as member button, then click Continue:
Then voilà - you're in!
NOTE: Once you're logged in as a site member, before you exit the site you MUST log out in the top right of the screen. Exiting the site while you're still logged in as a member can cause issues.