If you're previously screened a participant, then that participant will be in your account's database. You can access all participants through "Participants" tab in the top-level navigation.
To add an existing participant to an event, you can do it in 2 ways:
From within the event
1. Select the Event
2. Click "Add Participants"
3. Click "Add Existing Participant"
4. This will display a modal with a search bar
5. Enter at least 3 characters of the person's first name, or last name, or email or date of birth. This will trigger a search with matches:
6. Select the participants you want to add, and click "Add"
From the Participant View
1. Find the Participant in "Participants" tab.
2. Click "Add to Event"
3. This will present a modal with a search bar with a list of all Upcoming Events. Choose the event and click "Add":