How can I add people to an existing 1:1 conversation?

On mobile devices:

1. Tap a 1:1 conversation in the conversation list.
2. Tap your contact's name at the top of the conversation to show their profile.
3. Tap  (CREATE GROUP).
4. Type the names of the people you want to add and tap their avatars
5. When you are finished creating your group, tap CREATE.