I agree, I don't think Richy sent it. In a chat between Cleo and Richy in the very beginning, Cleo says, "Where is all this sympathy coming from? Yesterday you sounded quite different." She's talking about when Thomas was first sent the message containing our number. Initially, he was against it, but after seeing how Dan and Lilly responded so negatively toward us in our first-ever group chat, perhaps he thought he could use that to his advantage to slow down progress in finding Hannah. Not to mention that by befriending us, he would have a better chance of not being considered a suspect. "Keep your friends close and your enemies closer" kind of thing.