Apps to reach sources and how to organize contacts

Does anyone use Whatsapp or Signal to find or develop sources? I’ve heard of journalists joining WhatsApp groups to find people to talk to in disaster situations? How do you do that? You have to get invited? Is it worth doing?

And do you have a better way to organize contact info for sources you already have when working on a new story? Like could I group some of them all under a specific list and send a text to all in the group, but it’s not a group chat. Idk if that makes sense, but curious if you have any efficient ways to organize them!