Click Add more in the chat header to add new members into the modal that opens (figure 1):
Figure 1 Click Add more to launch the Add members modal
If you don’t see Add more and you’re not a room owner, the chat room preferences may allow only owners to add members. In that case, ask a room owner to add the members.
In Symphony 2.0 you can add members to a direct or group chat without losing the chat history. Learn more about group chats.