Dialog Messenger - handy and feature-rich enterprise multi-device messenger available for server or cloud – Slack-like, but not Slack-limited
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Ripcord - A desktop chat client for Discord and Slack
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Ziggs - Smoothly Share Content Between Devices!
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.