Add a new recipient to a group text

You can only add an another person if everyone in the group is using iMessage (blue). If anyone is using SMS, you can't add them.

Add and remove people in group text messages on your iPhone or iPad - Apple Support

1. Tap the group message that you want to add someone to.
2. Tap the group icons at the top of the thread.
3. Tap the gray arrow icon to the right of the contacts, then tap Add Contact .
4. Type the contact that you want to add, then tap Done