The fastest way to get in touch with investors you find on Signal is via mutual connections. If you connect your Gmail to Signal, you will be emailed mutual connections if there are any.
If you don’t have any mutual connections, the next best thing is to prepare a private BriefLink and email it to them so they can evaluate your company clearly. Only investors who see your link can see your Brief. It works like Docsend, but free and built just for startup fundraising.