Slack is a brilliant team messenger. But you wouldn't really use if for task management or file sharing. So you would quite quickly need to look around for other tools to handle this - and thereby spread your team thin across multiple apps.
SquidHub includes a team messenger and direct messages much like Slack. We just call Channels for Groups. But it basically works the same way.
Besides that, we offer a dedicated task manager for teams - as well as file sharing and a calendar.