I see questions about chat, communication, and forum solutions often enough that I figure I should write a descriptive post of several options to keep around for recycling. Since I don't know your exact needs I will describe three options here. Two options are very modern and has a lot of features and the third is very retro and simple text-based forum software with some cool privacy and federation features.
Delta Chat is a email-based chat app. I can use chatmail servers or email servers. It is end-to-end encrypted. (
https://delta.chat/en/). If your correspondents have POP/IMAP/SMTP email accounts they can use DeltaChat.
Delta Chat supports group chats and in-chat apps that you can create. It also now supports audio/video calls however they require using a link-based service.
If you want video and audio calls through your own XMPP server, or just private chat, XMPP is the way to go. Movim is a very mature server application for managing a social and chat network based on XMPP protocol. (
https://movim.eu/) Movim allows you to federate with other servers similar to how Mastodon servers federate, and it has more features not available in Mastodon, including video conferencing.
Movim has chat, blogs, chat rooms, and even screen sharing support.
If you want a public readable text-based forum without video or audio calls then Rocksolid Light is an option.
Rocksolid Light is a NNTP server with a web forum front end:
https://novabbs.org. You can run a private or public node, and even a private or public federated network.
Rocksolid Light has several themes included in the distribution. Here is the same server software running with a very different front end theme:
https://news.octade.net (username: 'guest', password is 'guest123' reversed)
You don't have to federate your Rocksolid Light server, but it you want you can federate it. If you federate it, you can send encrypted BBS mail messages between users on different domains. You can also compose encrypted PGP/Mime messages in a newsreader and post them encrypted to a newsgroup for that purpose, so your encryption keys don't have to reside on the server.
The bonus with NNTP is that users can read and post using a dedicated newsreader client that is threaded and has filtering and very compact message threading. You can also download all new messages at once and then read them and write the replies while offline.
Movim is a very highly developed option that includes everything including the kitchen sink, so if you want to grow into audio and video conferencing and federate blogs, it is a superior option. Movim has a very modern interface compared to Rocksolid Light. The advantage of Rocksolid Light is its old-timey simplicity for text-based discussion threads.
More about Movim is described here:
https://itsfoss.com/movim/.More open source WhatsApp alternatives are described here:
https://itsfoss.com/private-whatsapp-alternatives/#DeltaChat #E2E #Encryption #Movim #XMPP #Jabber #RocksolidLight #Servers #NNTP #Usenet #Forums #FreeSoftware #Newsgroups #NetworkNews #Retro #OldSchool
is finally there! 
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, and this year we will celebrate the 15th anniversary of the project 
BarbaPulpe 
ᴹᵃˢᵗᵒᵈᵒⁿ 
Nous promouvons le fédiverse. Vous aussi, devenez acteur d'un monde plus libre, éthique et non contrôlé par quelques entreprises capitalistiques !
Link to post in English: 