This plugin can be used to send a message to all users of the chatbot, the plugin handles correctly users using different platforms (Telegram, Messenger, Slack, etc.).
Import the Mass Contact sub-flow in Node-RED, then connect the output to all node senders supported by your chatbot.
Open Mission Control and type the message
If the chatbot is using multiple platforms (i.e., Telegram and Messenger) then Mass Contact will send the message using the right platform, just connect all the sender nodes to the Mass Contact sub-flow output. If a user is accessing the chatbot with more than one platform, the default one will be chosen (defined in the sub-flow configuration).
All contacts will be queued using the MC Queue node and sent to the platform at a regular speed, check the status of the queue clicking on the link "recipients queue" in Mission Control.
Installing plugins it's easy: open Mission Control (https://your-bot.com/mc or locally http://localhost:1880/mc) and then go to the plugins section and click on "install" (some plugins could include a Node-RED flow to import).
Be sure to enable Mission Control in RedBot / Node-RED and enable it in the bot configuration, read a complete tutorial here.