In the Messages app to choose contacts.
Note: If one or more of your recipients isn’t using iMessage, messages appear in green bubbles instead of blue ones. See to send the message.
People can respond inline as long as everyone in the group is using an iPhone, iPad, iPod touch, or Mac and has turned on iMessage.
Tap the group message you want to add someone to.
Tap the group photo at the top of the thread.
Tap Add Contact.
You can add someone to an existing group message (as long as everyone in the group is using an iPhone, iPad, iPod touch, or Mac and has turned on iMessage). Otherwise, you need to start a new group message.
You can remove someone from a group message as long as there are at least three people on the thread (and everyone is using an iPhone, iPad, iPod touch, or Mac and has turned on iMessage).
Tap the group message you want to remove someone from.
Swipe left on the name of the person you want to remove, then tap Remove.
Regardless of how many people are in your group thread, you can also Add or remove your phone number in Messages or FaceTimeWhat is the difference between iMessage and SMS/MMS?