There are a few ways to add a participant to your SignUp, depending on your goals.
If you're ready to invite people to your SignUp: Inviting people directly to your SignUp will add them to the Participant list of the SignUp. They will be able to choose their Spots (jobs/shifts/appointments) once you invite them.
Learn more here: How do I invite people to my SignUp? (Sending Personal Invitations is the first option in the article. If you invite someone using a Shareable Link, they will be added to the Participant List when they sign up and enter their own contact information.)
If you want to make assignments for people manually: Making manual assignments for Participants is easy -- you choose their Spots and they will be sent automated assignment notifications.
Learn more here: How can I add an assignment for a participant or volunteer?
If you're not yet ready to invite people, but want to gather a list of contacts: Create a Contact List as a holding list where you can add people to the list or edit it at any time. Later, when you're ready to invite people to your SignUps or Group Pages, you can invite them directly from your Contact List or make manual assignments from the Contact List. Learn more here: