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Answer the first call as you normally would answer the phone. As the next person calls, the person will pop up on your phone. Just accept the call and then press 5 to add the person to the call. Repeat the process of adding the next caller by answering the phone and pressing 5 until you've added everyone to the call.
Have the participants call your Google Voice number. Answer the first call. Add the next caller to the call. Continue adding callers until everyone is in the conference call.