You can add new players to your event at any time.
Before the event starts:
1. Go to My Events and select your event (or create a New Event)
2. Click "use pairing program"
3. On the Players Tab there is a text entry box
4. Type the name of the new player into the entry box
5. If the player is pre-existing in the system then select them from the dropdown list of players which appears
6. If they are a new player then Click the bottom option "Create new player named ..."
Boom! A new player has been added to your event.
If the event is in progress just click on the "Players" tab and you can add new players from the text entry box the same way as you would before the first round is paired. These new players will show up on the bottom of the pairing list and can be manually added into a the pairings if required.