There are two main ways participants can register to the event:
RSVP to your invitation
In this scenario you would typically import a list of participants you want to invite to the event, and send them an invitation email that contains a simple RSVP link. When they click on it it takes them to the event website where they can fill out the registration form (and pay if you have a paid event).
Accessing the event website
This scenario typically happens when an organizer has a public event and no specific list of participants to invite. You can promote the link to the event website on social networks, via your marketing campaigns and on your own website. Attendees will access the event website and there will be able to select tickets and fill out the registration form (and pay if you have a paid event).