Opt-in
A person’s agreement to receive WhatsApp messages from a business, which WhatsApp requires before the business messages them.
WhatsApp’s Business Messaging Policy requires a business to get a person’s permission before it starts messaging them, and to make clear which business will be messaging.
Good opt-in is easy to show: a sign-up form, a join link the person filled in, or the person sending a message such as JOIN themselves. MsgOrb records how each contact joined.