The conference calling feature is useful when you're on a call and need to add another person to it. To begin the conference call, you must be on a call. (It doesn't matter if you initiated the call or if you received the call.)
You can conference up to 16 people on your phone (including yourself).
To add people to a call in progress:
If you need to cancel the call because the person you're adding cannot join the conference call or the call rolls to voicemail: