iMessage got some new tricks in iOS 8 and one of those is the ability to add new people to an existing iMessage group. That means instead of having to start a brand new conversation, you can just quickly add someone to the existing group chat in just a few taps!
How to add a new person to a group iMessage thread
Before trying to add someone, remember that this works only if all participants in the thread are using iMessage. If the thread shows green bubbles instead of blue, you won't be able to add additional participants. You also can't turn an individual message thread into a group thread. It must be an existing group thread with at least 2 members.
And in case you're wondering, the person added will only see messages from the moment they are added on, meaning they won't see past messages from that thread.
Launch the Messages app.
Tap on the existing group message thread that you'd like to add someone to.
Tap on Details at the top.
Tap on Add Contact.
Choose the contact you'd like to add (you can add multiple people if you'd like) and tap Done.
The person will be added to the conversation instantly.