On smash.gg, you can add attendees directly towards the Brackets reporting page. This is particularly useful if you are only running one single bracket event and are adding people at the door.
Click Settings to access your Admin Panel
Scroll down and click on Brackets. Once there:
Switch to Edit Mode at the top:
Click on the Add Player button in the top right corner of the screen:
Enter the player's gamertag, adjust their paid status if necessary and click Submit.
While in Edit Mode, you can now drag and drop players to move them around the bracket.