Special Guests are characters who appear in your cafe at random intervals through the day.
These guests begin as people on the street (set apart by their names above their heads). You have to go through an 8 success street charm process before they will become regulars.
Once they become regulars, there are 5 levels of upgrading your conversations with them. Each level gives a new prize for reaching it. Level 5 being the best (either a clothing or furniture item).