You can manage the attendees without a payment mechanism by using the RSVP tool.
RSVP enables
First of all, go to Eventin -> Addons and enable the RSVP modules.
Auto Confirmation Email
You can enable auto confirmation for the RSVP events from Settings -> RSVP -> General Settings. You can also edit the mail body as you want.
User Allow to Form
You can allow users to access the RSVP form both with and without login.
Attendee limitation
You can set the attendee to how many attendees need to start the event.