How to add a contact to an existing text message thread in iPhone?

When you create a new group message (MMS group) on iPhone, you can add multiple contacts to it. However, once the group message is created and you're already in a conversation thread with those contacts, you can't directly add more contacts to that existing thread.

So, if you want to include another contact in the conversation, you have to create a new group message (MMS group) that includes all the existing members plus the new contact you want to add. Essentially, you're starting a new group chat with the same people as the existing thread plus the additional contact.

Here's how you can do it:

1. Open the Messages app on your iPhone.
2. Tap on the existing group message thread where you want to add a contact.
3. Tap on the group name or the "i" (info) icon at the top right corner of the screen.
4. Scroll down and tap on "Add Contact".
5. Select the contact you want to add from your contacts list.
6. Tap "Done" or "Add" to confirm.