You cannot as any user in the Sandbox (or any environment) unless you ask Salesforce support to enable the **"Enables Org Admins to log in as any user without having access granted by the users"** permission on your Org.

You can then turn the feature on by going to **Your Name** > **Setup** > **Security Controls** > **Login Access Policies** > **Administrators Can Log in as Any User**. Once that is done you should have the Login action link available for all users.

Otherwise you can ask the individual users to grant you login access by going to **Your Name** > **My Personal Information** > **Grant Login Access**.