With newer Wi-Fi routers you can set up a “guest” network; in other words, you can configure a separate name and password for friends, family, collaborators, visiting colleagues, etc. This feature is useful for people you don’t want to give your regular password to (so they can’t snoop around your personal files).
Guest networks generally include useful administrative features: from the router settings menu, you can see who is on your network, set an expiration time for their access and change the guest password when the current guest has logged out. Some routers even let you boot people off if they’ve stuck around too long.