I have no favorite community there but when it's my trip to join, I'm joining one since I've made up an account there but barely active. And usually, when the bot or any of the admins have welcomed and mentions you on the chat, it will notify you through your registered email there. So, that means that it's an automated notification for you every time that they include you to their post. I don't know the other features yet since I barely use it but that's one of my experience and have observed it for a long time.
To give you an idea about signals, there could really be a lot of people that are interested on it. But in reality, it's not worth it when you're already part of it in a long time and much worse if they'll ask you to pay for subscription or just to not be kicked out of the group.