Site administrators can log in as any user on the site besides another user with the 'Site Admin' role. This features is known as "masquerading" as a user.
Log into the site as a user with the site admin role.
Find the user to masquerade as and view the user profile.
Scroll down to the bottom of the page and click "Masquerade as [user]."
If SSO is implemented, the Masquerade feature may not work due to the custom logic in place.