iOS has two different mechanisms for sending messages, iMessage and SMS/MMS.
iMessage is a communication service provided by Apple that can be used only on Apple devices. SMS and MMS are normal text messages that work through your cellular provider and can be sent to non-Apple devices.
Group iMessages
When you create a group message, if all of your recipients use iMessage, then after you tap Compose in Group Text+, the title will display "New iMessage" and the recipient names will be colored blue. iOS will then send a group iMessage: every recipient can see each other, and all replies are sent to every member of the group. The only way to prevent this behavior is to disable iMessage altogether. To do so:
- Open the iOS Settings app from your homescreen
- Tap Messages
- Disable "iMessage"
MMS Group Messages
If you create a group message and not all of your recipients use iMessage, or you have iMessage disabled, then after you tap Compose in Group Text+, the title will display "New Message", and the recipient names will be colored green. By default, iOS will send an MMS group message through your cellular provider's network. Different providers impose different limits on how large an MMS group message can be; some providers' limits are as low as 10. (You can contact your provider to determine how large a group you can send to.) Like a group iMessage, every recipient can see each other and all replies are sent to every member of the group.
To disable this behavior and instead send individual messages to each recipient in the group:
- Open the iOS Settings app from your homescreen
- Tap Messages
- Scroll to the "SMS/MMS" section and disable "Group Messaging"
Keep in mind you must also have iMessage disabled as above if all of your recipients can receive iMessages; otherwise, they will receive a group iMessage regardless of this setting.
If you are having issues sending messages through iMessage or SMS/MMS, or would like to read more about how they work, Apple provides some more documentation: