I would not assume a group whiteboard will continue to exist, so I would either archive it in a more durable image format and just include the image in your document, or archive the whiteboard on the wayback machine and cite it from there (google that to find out how; it's a common question). In general, if I were grading essays that included links to random web services I would be wary of them. Think about it this way: the group work is the whiteboard itself, not the mural.co service, so you want to safeguard and cite that group work in an as durable way as possible.