When the Church comes together in the liturgical assembly to celebrate the Mass, or any other sacrament, her members do not gather simply as a crowd, as an amorphous, undifferentiated group of people. They gather in a variety of ministries and roles.  The Constitution on the Sacred Liturgy of the Second Vatican Council tells us that participation in the liturgy is the right and duty of all the baptized. 

In addition to the ordained ministries there are roles in the liturgy which are exercised by lay people who place their time and talent at the service of the liturgical assembly as altar servers, lectors, extraordinary ministers of Holy Communion, cantors, choir members, instrumentalists, leaders of song and ushers. Others contribute their time and talent to planning and organizing the liturgy, to keeping the church and the vestments, vessels and appointments clean and well-ordered or to providing decorations that reflect the spirit of the liturgical feast or season.