Slack is a great collaboration & communication tool: I found it esp. useful when you want to maintain a repository of info around a single subject, i.e.
- a product launch team can post meeting notes, spreadsheets, and all the launch-related info the team creates and discusses.
- a company's re-branding task force might keep all survey data and art files in one place, along with comments on each and discussion threads on the project.
Plus it's very easy to use. Whoever initiates a new "channel", invites the relevant participants.