Open the Messages app
Tap the New Message icon
Enter the names or phone numbers of the people you want to add
Type your message
Send the message to create the group chat
To add more people later, open the group chat
Tap the group name or icons at the top
Tap Info
Tap Add Contact
Select the person you want to add
Tap Done
Use iMessage for full group chat features
Make sure all group members have iMessage enabled for best results