To set it, tap the i button in the top right corner of your chat window. Someone in a chat thats spamming way too much. Tap the group conversation you want to add someone to. Therefore, this will not work for the two parties conversations where you want to include a third person.
If you wonder how to restore disabled iPhone without iTunes, or restore your iPhone in recovery mode, follow the steps with iOS System Recovery as this. Enter the contact information for the person you want to add. Most importantly, before you start using this method, note that the tutorial we are about to unleash only works for the Group Chats. How do I add people to an already existing group chat.