Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
PartyWith - Connecting people who love to party - anytime, anywhere.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
LinkedIn Events - A new way of connecting people from LinkedIn
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Clearbit for Slack - Send all signups directly to a Slack channel