That's really not such much a 'mod', but just a setting. In the System Preferences App, select the Accounts tab. In that panel click on the Login Options button (you may have to click the lock at the bottom of the panel and enter your password first). Then select 'Enable Fast User Switching', and set the View As: pulldown to name.

Those settings get your name on the menu bar, and when you click your name, there will be a pulldown of the names of all the other account holders on the system. Selecting one will bring up a login panel, and that user can login, leaving the existing user logged in as well...