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.
Start to type a name in the text field , or click the Address Book icon  to filter by users or course groups.
Verify you added the correct person.
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.