Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Event Hunt - Startups and Tech Events Curated By Locals
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
Mobilizon - A free and federated tool to get our events off Facebook
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.