pleroma/docs/ap_extensions.md

ChatMessages

ChatMessages are the messages sent in 1-on-1 chats. They are similar to Notes, but the addresing is done by having a single AP actor in the to field. Addressing multiple actors is not allowed. These messages are always private, there is no public version of them. They are created with a Create activity.

Example:

{
  "actor": "http://2hu.gensokyo/users/raymoo",
  "id": "http://2hu.gensokyo/objects/1",
  "object": {
    "attributedTo": "http://2hu.gensokyo/users/raymoo",
    "content": "You expected a cute girl? Too bad.",
    "id": "http://2hu.gensokyo/objects/2",
    "published": "2020-02-12T14:08:20Z",
    "to": [
      "http://2hu.gensokyo/users/marisa"
    ],
    "type": "ChatMessage"
  },
  "published": "2018-02-12T14:08:20Z",
  "to": [
    "http://2hu.gensokyo/users/marisa"
  ],
  "type": "Create"
}

This setup does not prevent multi-user chats, but these will have to go through a Group, which will be the recipient of the messages and then Announce them to the users in the Group.