Riot.im is an open source Android/iOS/web messaging app. It uses the Matrix protocol, which is also open. There is a reference Matrix server called "Synapse", which I actually just started running the other day.
With these components, you can host your own messaging platform.
I use it as an IRC/Slack alternative, and it's pretty good - especially for the equivalent of public channels. The apps are better and easier to explain than IRC ones, and it is easier to join and more open than Slack.
Naturally, if you're making a new channel for a group, you have to ask everyone in the group to join it. That's the case whatever messenger you use.
There is crypto now for private messages, but it is a little clunky to setup still. I hope that the funding helps make this better!
For friends I've shared keys with, it works really well. Of course, right now people will only join for ideological reasons. That's OK though!
I'm connected to Discord, Slack, and irc chats from Matrix/riot.im. It's working great for me after about a year of use.
I also invited some friends to join my Matrix server directly, but there wasn't much pickup yet. But that doesn't matter since I'm already connected to the places where people are via Matrix.
Check voyager.t2bot.io for a big list of different public Matrix rooms. There are lots of active rooms, as well as bridges to the biggest IRC networks, Slack, Gitter, Discord, and more.
With these components, you can host your own messaging platform.