The challenge with group prayer is that a prayer of agreement requires everyone to agree in the specifics of the prayer, but each person must also be in right standing with God. No un-forgiveness in their heart, no unconfessed sins and the agreement must be specific.
Consider a chorus. When everyone sings in pitch the right way, it can be beautiful, but if one person or more is out of tune, it ruins the experience. Such is the case with God and many people praying to him at once. The more that pray together who are in right standing, the more powerful the prayer is before God the more dynamic the results.
This is the difference, exponential voices are like a chorus and God loves a chorus.