From inside of the group chat, tap sign, then select 'Add Members'. Select the new members and tap Save to return to the chat. The person will be automatically added to the Group Chat and the Chat Group. (Only the Group Admin can add members).
When you receive a new Voicemail from someone who is not an existing contact, you can add their details to your device either by: Tapping on the Tendant home screen and then selecting the conversation displayed as 'Unnamed Caller' where you can tap ...