Can I add / remove contacts from a group text message?
UPDATE: iOS 8 brings some of this capability to the Messages app. To add a contact to an existing group text message follow these steps in Messages:
1. Open the group message thread
2. Tap Details in the upper right corner
3. Tap Add Contact to add another party to the group message
UPDATE 2: You can now delete a person who you added to a group message by going to the "Details" panel and swiping on their name. Learn more about adding and removing contacts with iOS 8 here.