When the members of the Body of Christ come together in the liturgical assembly to celebrate the Mass, they gather in a variety of ministries and roles.  The first qualification for any participant in the celebration of the Eucharist is that he or she has been baptized into the Body of Christ.
In addition to the ordained ministries (bishops, priests, deacons) 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, cantors and choir members, ushers, and extraordinary ministers of Holy Communion.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.
All the baptized need to understand that part of their duty regarding liturgy is to accept some responsibility for the liturgy, to place themselves and their God-given talents at the service of the liturgical community whenever possible.  Whether one brings up the gifts at the Offertory, reads the Word of God, assists with distribution of Communion, serves at the altar, provides music that augments the joy, solemnity, and festivity of the celebration, or serves the assembled community as an usher, he or she is contributing to the worship of the community and fulfilling the responsibility that comes with Baptism.