The liturgy is the public worship of the Church. It is celebrated, not by individuals or groups within the Church, but by the entire Body of Christ.
All of the faithful are called to fully participate in the liturgy. Most of us do this by coming to the liturgy spiritually prepared, ready to join our minds and hearts, indeed our bodies, to all that we as a Church hear, say, and do in the liturgy. In this way, we cooperate with grace.
However, there is need of some of the faith ful to take up special, integral roles in the liturgy, such as altar servers, sacristans, musicians, lectors, greeters, and extraordinary ministers of Holy Communion.
Photo credit: Joe Garcia of eCatholic.com