If I analyze it correctly, first of all I have the api already for Whatsapp.
both can receive and send message from whatsapp, as it running on server (linux,posix). its act as "sender" for both send and receive.
then second part is must be, email part. an single email, for receiving message from your limited users. act as bot to receive, then convert it to API whatsapp, to send.
the simple way maybe is just use Subject as Number Phone of whatsapp (for Sending). and when receiving message from whatsapp, we send back to email user, based on something smart enough (because this is multiple user)
we can discuss more,
pm me