Hello Heraveee,
Thank you for using Apple Support Communities.
I see that you are having difficulties in sending group messages from your iPhone. I have a couple of articles for you to take a look at to sort out what might be going on. It sounds like you are not unfamiliar with group messaging, but I would suggest starting with the first on sending group messages, to ensure that you are not omitting something necessary.
Send a group message with your iPhone, iPad, or iPod touch
After that, take a look at the steps in the following to eliminate other issues on sending/receiving messages.
If you can‘t send or receive messages on your iPhone, iPad, or iPod touch
Best regards.