When a participant has been applied to participate in your project, but hasn't yet been invited, Their status is "pending" and they show up in your account as either "Qualified" or "Not Qualified."
In the example below, Jack is a pending participant and showing up as Qualified:
Invite - You can invite pending participants to complete your project by clicking the "Invite" button to the right of their name. When you invite participants their status changes to Invited.
Hide - If you don't want to view the participant anymore, you can hide them by clicking the "Hide" button. Here's a bit more on hiding the participant.
See more on the other participant status here.