105. A liturgical function is also exercised by:
a) The sacristan, who diligently arranges the liturgical books, the vestments, and other things that are necessary for the celebration of Mass.
The commentator is still on board. But as the years go on, I find there’s less and less of a need for this role:
b) The commentator, who, if appropriate, provides the faithful briefly with explanations and exhortations so as to direct their attention to the
celebration and ensure that they are better disposed for understanding it. The commentator’s remarks should be thoroughly prepared and notable for their restraint. In performing this function the commentator stands in a suitable place within sight of the faithful, but not at the ambo.
Ushers, greeters, ministers of hospitality, or whatever term is used:
c) Those who take up the collections in the church.
d) Those who, in some regions, welcome the faithful at the church doors, seat them appropriately, and marshal them in processions.
The parish liturgist, too:
106. It is desirable, at least in cathedrals and in larger churches, to have some competent minister or master of ceremonies, to see to the appropriate arrangement of sacred actions and to their being carried out by the sacred ministers and lay faithful with decorum, order, and devotion.
107. Liturgical functions that are not proper to the Priest or the Deacon and are mentioned above (nos. 100-106) may even be entrusted by means of a liturgical blessing or a temporary deputation to suitable lay persons chosen by the pastor or the rector of the church.[Cf. Pontifical Commission for the Interpretation of Legal Texts, response to dubium regarding can. 230 §2: Acta Apostolicae Sedis 86 (1994), p. 541] As to the function of serving the Priest at the altar, the norms established by the Bishop for his diocese should be observed.
It’s actually hard to envision the clergy shouldering most of the roles of GIRM 100-106, at least on a regular basis. In most parishes, one priest has other duties, and even in small parishes, the fine details of the liturgy are usually well-covered by the laity.
I will mention at this point that it’s optimal for a person to be discerned and well-prepared for any regular or permanent liturgical ministry. That person may well be prepared for more than one role, but I think the “professional” volunteer is not the ideal way to conduct good liturgy–as a rule. If the roles of GIRM 100-106 are to be taken seriously, they need to involve people who make a point of preparing and planning for the Mass in a singular and focused way. This reinforces the Pauline principle of spiritual gifts, and also underscores the hierarchical nature of liturgy. A priest does priest things. A musician does musical things. A lector proclaims. A Communion minister serves. A sacristan organizes. A greeter greets. An usher ushes. A Catholic serves in the role for which she or he was discerned, recruited, and prepared.