If your group texts are not sent individually, there are two possible reasons for that :
- They are sent through iMessage(in blue) : By default iMessage handle group texts as group conversations. To avoid that, just turn off iMessage and delete any previous group texts sent with iMessage ON before sending a new group text through the app.In case you absolutely need to keep iMessage turned ON, you can use the Personalized Texting option.
- They are sent through MMS(in green) : Some carriers are detecting group texts and if MMS is ON, group texts will be automatically handled as conversations. To avoid that, you can ask your carrier to disable this option or just turn off MMS in iPhone messages settings. Be aware that turning off MMS will then not allow you to send pictures in attachement.
More information about this topic can be found on Apple's website : Send a group message on your iPhone