The quote below from http://support.apple.com/kb/HT5760 indicates that it in the situation you describe it should be going as MMS to everyone -- which is not what you are seeing actually happen (and also not what you want to happen if I understand correctly). There may be something else helpful in the link.
Group messages will be sent using iMessage if all recipients have iMessage enabled. If not, the conversation will be sent as MMS. Group messages use MMS even if the content is text only.