Rocket.Chat/app/discussion/server/hooks/joinDiscussionOnMessage.js

import { Meteor } from 'meteor/meteor';

import { callbacks } from '../../../callbacks/server';
import { Subscriptions } from '../../../models/server';

callbacks.add('beforeSaveMessage', (message, room) => {
	// abort if room is not a discussion
	if (!room || !room.prid) {
		return message;
	}

	// check if user already joined the discussion
	const sub = Subscriptions.findOneByRoomIdAndUserId(room._id, message.u._id, { fields: { _id: 1 } });
	if (sub) {
		return message;
	}

	// if no subcription, call join
	Meteor.runAsUser(message.u._id, () => Meteor.call('joinRoom', room._id));

	return message;
}, callbacks.priority.MEDIUM, 'joinDiscussionOnMessage');