How do I add someone to an already existing text chain

Hi there AEHS,

Welcome to Apple Support Communities. We're happy to provide some insight into adding someone to an existing text thread. You do have the ability to add someone to an existing iMessage thread. However, you cannot add another person to a group MMS or SMS text thread. You can find more info on that here:

Add and remove people in group text messages

Feel free to reach back out if you have any questions.

Best regards!