Why did someone see that I joined Signal? People who already know your number and already have you in their contacts see that they can contact you on Signal. Nothing is sent to them by your Signal app or the Signal service. They just see a number they know is registered. If someone knows how to send you an insecure SMS, we want them to see that they can send you a Signal message instead.
Why did I see that my contact joined Signal? You are notified when someone that is stored in your contact list is a new Signal user. If you can send an insecure SMS to a contact, we want you to know you can send a Signal message instead.