I think the only way around this is to set up cell phone emails instead of sms's if it's always the same group of people. I have a group of 15 cell phones I send to during snow storms. I use , etc. etc. It's an sms via email basically.
With the 8700g handheld OS v4.1.0 you can't SMS to groups; you have to do it individually or via e-mail to the phone e-mail address.
If you upgrade your 8700c to the 4.2.1 handheld OS, then it IS possible. I currently have several SMS groups set up this way. It makes it much easier and is one of the reasons I upgraded. For isntructions on upgrading the handheld OS, refer to the sticky on the top of the 8700 forum.