You should be able to create a list (or group) in your messages tab. Then when you want to send out a message to everyone in the group, you just have to send it one time. I hope you find this feature helpful. It works for me.
That really is the best route. Originally, FB groups didn't have that feature, which really made them useless. People only saw group info if they went to the group's page. Now, it works pretty well. Any msg will go to all FB users in the group unless they have opted out of receiving msgs from groups in general or that group in particular.