We are using the nrf52840 with OPENTHREAD and would like to use the Nordic COAP application because block transfer and observe are implemented. We need to do multicast and group communication. Does the Nordic IoT COAP application support multicast communication for discovery and data transfer like is described in RFC7390 group communication for COAP?
Unfortunately, IoT CoAP has a limitation, that client will process only the first response for a multicast request, so group communication scheme you've mentioned will not work as expected.
As a workaround, you could specify a resource on a client side to gather responses, and send a response as a separate transaction (CoAP request).