The organizer has the ability to manually specify the default visibility after the participants register. This can be done in Configuration - Registration Settings - Participants Visibility After Registration. This feature is used by some organizers when they want to hide the participants list on the website until a certain number of participants are registered.
Participants are automatically visible
This is the default option for your events and it means that the participants will be visible to one another and, if the event is set to public, to the website visitors too, immediately after they register.
Manually turn participants visibility on
Participants will only be visible for other participants once their visibility has been manually turned on by the organizer and once they are activated.