Eventbrite is a popular social platform that allows event organization for free. If your event has ticket-related costs then they receive a small portion of profit off of the transactions that each attendee makes.

Meetup.com works a little differently. In order to create an event you must be a part of a group, but there is a cost associated with creating that group. I don't believe there is a limit on members of groups, but there is an application process each group has to filter members.