My guests are being asked to log in, and I don't know why!

You may have accidentally enabled Advanced Guest Management, which allows you to upload a list of guests, restrict the # of +1s they can bring, which events they are allowed to see information on and RSVP to. If you want to go back to Simple mode, where all guests see the same version of your website and the same RSVP, you can go back by clicking the link at the bottom of the page at "Guest Management" on the "Guest List" tab.