adding a contact to a group message
Info on internet does not show any of the helpful hints in adding a contact to a group message with version 15.2.1 - I hate to remove and set up a new one.
Tap on the conversation. Touch on the icons of participants at the top. Touch on the number & list of participants towards the foot of the screen. Scroll down a little and you will see a + sign inviting you to Add Contact (ie. a new participant) to the group chat.