The first possibility to find your relatives on Signal is to invite them individually using the module provided for this purpose within the application.
To do this, open the application and press your avatar to access Signal's settings. There you should find an Invite your friends option.
When you press it, Signal will offer you to send an invitation by Message (SMS) or e-mail. Your loved one will then receive the link to download the application on their smartphone.
Prefer the SMS invitation rather than the e-mail invitation which requires that the contact form of your loved ones contains their e-mail address, which is not always the case unlike the mobile number.
Note that you can also invite your friends and family by pressing the pencil icon (like writing a new message) and then selecting the Invite friends to Signal option.
Signal offers another possibility to invite your contacts to join the application. You can contact a WhatsApp group directly by sending them a direct link that will allow them to join the same newsgroup, but on Signal, as long as they are registered on the application.
To do this, open Signal, press the pencil icon to initiate a new conversation and choose to create a New Group. Without selecting a member for the moment, press the Next button directly, fill in the name of your group and validate it by pressing Create.
In the conversation that appears, press the name of the group to access the options. Then enter the Group Link menu and activate the option with the same name.
Now press the Share button and choose the option that suits you best. You will then be able to share the link directly from the Signal group to the appropriate WhatsApp group, or copy the link and send it in some other way to the people you want to join the group.