Awesome Privacy

Easy-to-deploy, self-hosted team collaboration platform with stable, feature-rich cross-platform client apps. The UI is fast, good looking and intuitive, so very little technical experience is needed for users of the platform. Rocket.Chat's feature set is similar to Slack's, making it a good replacement for any team looking to have greater control over their data.

Secure group communications, with the option to be used over Tor or I2P. Fast intuitive group and 1-to-1 chats with text and rich media using decentralized chat rooms, with a mail feature for delivering messages to offline contacts. A channels feature makes it possible for members of different teams to stay up-to-date with each other, and to share files. Also includes built-in forums, link aggregations, file sharing and voice and video calling. RetroShare is a bit more complex to use than some alternatives, and the UI is quite retro, so may not be appropriate for a non-technical team.

Privacy-focused messenger using the Matrix protocol. The Element client allows for group chat rooms, media sharing voice and video group calls.

An IRC-based solution is another option, being decentralized there is no point of failure, and it's easy to self-host. However it's important to keep security in mind while configuring your IRC instance and ensure that channels are properly encrypted - IRC tends to be better for open communications. There's a variety of clients to choose from - popular options include: The Longe (Web-based), HexChat (Linux), Pidgin (Linux), WeeChat (Linux, terminal-based), IceChat (Windows), XChat Aqua (MacOS), Palaver (iOS) and Revolution (Android).

Mattermost has an open source edition, which can be self-hosted. It makes a good Slack alternative, with native desktop, mobile and web apps and a wide variety of integrations.