Hi,
Is it approved in the SCCM Admin console? Workgroup computer are per default not approved, there is a site setting you can change to auto-approve all computers and not only domain join.
Find site will query AD for a site to belong to so it will not work. Network access account is required, is that configured?
Check out this post as well.
http://eskonr.com/2013/08/sccm-configmgr-2012-manage-workgroup-computers-for-deploymentremote-tools-etc/
Regards,
Jrgen