Mattermost, opewn-souce clone of slack
There is an open source clone of slack called mattermost. It's seems ok, I'v spun it up and played with it for half an hour. I got no futher because the standard way of using it seems to be to use docker. I havn't had the enthusiasm to spend a day learning docker yet, I would have no idea how to fix the mattermost container if it just stopped booting, how to keep it backed up and patched.
Slack costs hundreds of dollars a month for a couple of dozen users. The cost is high enough that getting someone to set up mattermost is a reasonable alternative.