All Liturgical celebrations, especially the celebration of the Eucharistic Liturgy, are celebrations of the whole Church. They are not private functions. The Church is the “sacrament of unity: the holy people united and arranged under their bishops. Therefore, liturgical services pertain to the whole Body of the Church”.   (CSL 26)

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 acolytes (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.
The General Instruction makes it very clear that this variety of offices and roles is desirable and should be maintained. There is a wide variety of services to be performed, and it is desirable that different individuals exercise those services so that the talents and gifts God has placed within the Christian community are fully utilized and these roles of service are not monopolized by a few. 

The liturgy, then, is about the action of God's own people, each with different offices and roles, each office and role, from that of bishop and priest to that of usher and sacristan, one of service, not of privilege, a mirror of Christ who washed the feet of his disciples and instructed his followers to imitate his example of service. (USCCB)