You can add people to an existing thread in the Conversations Inbox.
Note: When adding users to an existing conversation, you cannot change the course or the subject line.
In Global Navigation, click the Inbox link.
In the Inbox filter, select an existing Conversation.
Reply to Conversation
Locate the message thread and click the Reply icon.
Start to type a name in the text field , or click the Address Book icon  to filter by users or course groups.
Use Address Book
Locate the individual and select his or her name . Names are sorted by last name.
To navigate back to the course roster, use the arrow icon .
To exit the course roster menu, press Return (on a MAC keyboard) or Enter (on a PC keyboard).
Verify you added the correct person.
Enter a reply in the text field . Then click the Send button .
The link will update the total number of people involved in the conversation.
Newly added participants will be able to read and comment on the entire Conversation, even the messages that were posted before they were added.