Slack started as a company called TinySpec which created a massively multiplayer game called Glitch, and weren’t getting the growth in the game that they were looking for.
The game server had a bot which indexed all messages that had been sent. Users were using the builtin IRC server for messages. The developers pivoted and came up with the idea of using the IRC server as a standalone product; SLACK, with a backronym of Searchable Linked Archive of Company Knowledge. Group messaging succeeds if the users feel like they are part of a shared space.
Keith Adams on the Architecture of Slack, Using MySql, Edge Caching, & the Backend Messaging Serverpage