In order to attend a free plan you just need to click on the “Join the list” button in the description section of the plan.
If you want to increase the number of friends that are coming with 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.
To attend these events, you don't need to show the ticket at the entrance. Remember that management can refuse service to anyone at any given time. Review also the confirmation email for further details.
In order to check if you have joined the list, just visit the "Tickets" section in your profile.