 <p>I'm still not sure if this is doable through the REST endpoint. We've found cases where the SOAP endpoint offers functionality that the REST endpoint does not, and this might be one those cases.</p> <p>What I did was to use the SOAP endpoint instead. There was no built in method for this, so I had to use the general one. That, in this case, was an organization request, where I had se the request name to "AddMemberList".</p>