Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Ripcord - A desktop chat client for Discord and Slack
Bigtent - BigTent is a web-based organization and communication portal for groups.
Ziggs - Smoothly Share Content Between Devices!