Never mind, I found the registry value.
It was in
HKLM\System\Inbox\Settings\LimitSMSRecipients
I changed the decimal value to a higher number
By the way, if you are looking for a GREAT little app to send group texts, GroupSMS is amazing (
http://www.logic-unlimited.com/groupsms.php). It sends to groups you have defined in your Outlook.