Theres a number of options
If the joining mechanism is that users go to the bots own channel and writes !join then you can read that from chat.
If the joining mechanism is a website then you need to devise a way to send messages to a running instance of the bot. Personally I use Redis PubSub to send messages in/out of a running instance.
But you could also add the user to a database and have the bot check the database for changes every minute or so.