Discord.app was created as a collection of sets of chat channels (each set is called a "server", and I have a server named Smart_Token, and I have not yet made any special channels inside it) to serve gaming communities! It has been so succesfsul that the owners have extended its reach to all kinds of communities.

It is free to join. When you do, visit the SteemitBC "server" to see a fleshed-out set of channels, one of which is crypto-trading, and another is crypto-taxation.

Stuff submitted to a channel can be seen by the public; but there ius a DM (Direct Message) service where private messaging happens.

Steemit set up a Steemit.chat service; but Discord is much more powerful. It even allows voice chatting.