User Activity Log Pro is a widely-used WordPress plugin to track all user activity and get email notifications after detecting suspicious activity. Also supports WordPress multi-site functionality. You can observe all the modifications that are made by other users such as creating and updating posts, pages, media uploads, etc. and admin can logout that particular user from any device as he wants for security. You will also get an email notification when selected activities are done on site. You will get more functionality if you install this plugin.