You can RSVP for an event by going to the event details page and then click on the "I'm going" button.
If you have one or more people of your friends or family who would absolutely love to volunteer for an event with you (and your organization allows you to do so), you can add them as guests when you rsvp for yourself. All you have to do is go to the event and click on the "I am going" button on the event page.
Note that if you are the creator of the event, or you have already RSVP'd for the event, you would have to first remove your RSVP from the event and then again RSVP to add guests. If you have already RSVP'd, you will see a button with label "Can't make it" instead of "I'm going". Click on the can't make it button and you will see the attend button. Once you click attend, you will see a popup which will allow you to add guest users to your rsvp.
Below is a walkthrough video for you to add guests to an event.