Scroll down to see how to do this in Symphony 1.5
In Symphony 2.0:
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.
In Symphony 1.5:
Click the i in the top right of your chat to add people using Chat Details (Figure 1):
Figure 1 Access Chat Details Information
Open the Members section, click + Add Members, and a modal will appear. Select the names of contacts you wish to add, and click Add (Figure 2):
Figure 2 Add members modal
If you don’t see this option, you may not be a room owner, or the chat room preferences may be set to only allow owners to add room members. If so, ask the room owner to add the required members.