Chapter III-B. Offices and Function of the People of God
62. In the celebration of Mass the faithful are a holy people, a people God has made his own, a royal priesthood: they give thanks to the Father and offer the victim not only through the hands of the priest but also together with him and learn to offer themselves.  They should endeavor to make this clear by their deep sense of reverence for God and their charity toward all who share with them in the celebration.
They therefore are to shun any appearance of individualism or division, keeping before their mind that they have the one Father in heaven and therefore are all brothers and sisters to each other.
They should become one body, whether by hearing the word of God, or joining in prayers and song, or above all by offering the sacrifice together and sharing together in the Lord's table. There is a beautiful expression of this unity when the faithful maintain uniformity in their actions and in standing, sitting, or kneeling.
The faithful should serve the people of God willingly when asked to perform some particular ministry in the celebration.
63. The schola cantorum or choir exercises its own liturgical function within the assembly. Its task is to ensure that the parts proper to it, in keeping with the different types of chants, are carried out becomingly and to encourage active participation of the people in the singing.  What is said about the choir applies in a similar way to other musicians, especially the organist.
64. There should be a cantor or a choir director to lead and sustain the people in singing. When in fact there is no choir, it is up to the cantor to lead the various songs, and the people take part in the way proper to them.