To add a participant to a program, you need to have invitations available in your account. You can see invitations in your account here:
There are two ways to add invitations:
Click on Get more invitations next to your invitation counter:
Navigate to Company settings then click the Add invitations button
You can then select the number of cards you want to add to your account, click the Continue button and complete the payment process. The invitations will be added to your account.