No process adds all the contacts in your address book to Signal. Rather, each user must register voluntarily. If you want to talk to others through this messaging platform, your only option is to invite them and wait for them to create their own profile. Remember that you can also check which contacts already have Signal and activate a notification to alert you when one of your contacts signs up. Let us explain it all to you in this article.
How to invite others to use Signal
Inviting other users to Signal is very simple. The first step is to click on the blue button located in the lower right corner of the main screen. Its icon represents a pencil and is used to start new conversations.

Then, scroll to the bottom of the contact list and tap on Invite Signal.

Customize the message, but do not remove the link. Tap on Share.

Select how you want to share the message. Once your contact receives it, they will need to install and configure Signal on their own.

Once the profile has been created, you will be able to start a chat with your friend using Signal.
How to know the contacts that have Signal
Every time a contact from your address book creates their account in Signal, they automatically appear in the application. To see which contacts are available in Signal, tap the button to start a new conversation.

Then, take a look at the contact list. Everyone on it has a Signal account.

Tap on any of them to start a conversation.
How to enable new contact notification in Signal
Although there is no automated way to add new contacts to Signal, you can receive a notification each time one of them joins Signal. Go to the application settings and tap on Notifications.

Scroll to the bottom of the list and activate Contact joined Signal.

Now, every time you have a new contact in Signal, you will be notified.