How To Add a New Contact To Group Message on iPhone 11/iOS 14
If you have updated your iPhone to iOS 8 or later, you may have sent group messages on your iPhone. When you chat with friends by messages and want to add a new friend into this group, you may not know how to make it. Here, in this guide, we will show you how to add a new person to a group message on iPhone.

However, what you'd better keep in mind is that the method mentioned below works when everyone in the group is using iMessage. Besides, the person added in the group can only receive the messages from the point he or she has joined.
Steps To Add a New Contact To Group Message on iPhone 11/iOS 14
Step 1. Open the Messages app.
Step 2. Find the existing group message you want to add a contact.
Step 3. Tap "Details".
Step 4. Tap "Add Contact".
Step 5. Choose the contact you want to add, and select "Done".