1. To add someone to an existing chat tap on the names of the existing members to get to the chat menu.
2. From the menu select 'members'.
3. You can then add members by tapping on their name/picture. Members you have already added will appear greyed out.
Just start typing the person's name into the search bar and when you've found them tap on them, to find someone else tap the search bar again and type in someone else's name.
Once you've added everyone you want tap the pink tick to start the conversation.