In order to attend a free event you just need to click on the “Join” button in the description section of the event.
You should be careful not to click the button twice because it will make you leave the list.
If you want to increase the number of friends that are accompanying you, you should go to the plan details and click on "Leave event". After that, you can join the list again selecting the number of attendees you want. In order to check if you have eventually joined the list, just visit the "Tickets" section in your profile.
To attend these events, you don't need to show the ticket at the entrance, but remember that management can refuse service to anyone at any given time. Review also the confirmation email and the ticket information for further details.