You can add participants to your event in the Participants module, at Event level. There, click on the + button to start adding participants, you can choose to add a single participant or to upload a spreadsheet.
To add a single event participant, select that option and fill in the required basic details (First name, Last name and Email address). When you are done, click on save and the participant will appear in the list of existing participants.
To learn how to upload a list of participants at once, please read our article How to import a participant list?