No reservations,they're free with EPCOT admission.For the most popular groups you'll have many people in line as early as 3pm or earlier for the first show which is usually at 5:15pm,so the first show tends the be the most crowded.The later shows are at 6:30 and 7:45pm.It's true that the theater does hold a lot of people but there have been groups that fill it up, for example I remember seeing the Beach Boys one day and people were waiting at 2pm and by 4:45 or so the line went into Italy then snaked back into the American pavillion and snaked back to reach Germany,there's no way the people near the end got in,then the next day the Family Stone was there,we walked into the theater about 15 minutes before the first show and got decent seats.