Your event page's RSVPs will be open by default — no need to publish your event for it to be live. As soon as you're done preparing your event, you can share your event and people will be able to RSVP. You do not need to take any other actions initially for them to be able to do so.
Want to make sure? Just hit the Preview Event button (the one that looks like an eyeball) near the top right of the Event Settings page. You'll be able to then see your guests' view of your event page, RSVP button included.
If you've previously closed your RSVPs, and you now need to re-open them for whatever reason, simply:
1. Jump into your Settings tab from the Event dashboard.
2. Select the Registration Closed drop-down and change to Registration Open.