I've attached my APN settings and the Group MMS setting tickbox. Being on the "phone" APN is not required for group mms, and wap.cingular APN uses a different MMSC gateway so don't mix the settings with other APNs as it won't work. Either way, this feature worked on both APNs on AT&T so I don't know. Can you post a screenshot of the greyed out tickbox?