Open the group text
Tap the group name or info icon
Select Add Contact or Add Member
Choose the person from your contacts
Tap Done or Add
If you do not see an add option, start a new group text and include the person
On iPhone, use iMessage group chats to add people only if all members use iMessage
On Android, use the Messages app group settings to add participants if supported
If the app does not allow adding people, create a new group with everyone included