18 The faithful are called to engage in the apostolate as individuals in the varying circumstances of their life. They should remember, nevertheless, that man is naturally social and that it has pleased God to unite those who believe in Christ into the people of God (cf. 1 Peter 2:5-10) and into one body (cf. 1 Cor. 12:12). The group apostolate of Christian believers then happily corresponds to a human and Christian need and at the same time signifies the communion and unity of the Church in Christ, who said, "Where two or three are gathered together in my name, there am I in the midst of them" (Matt. 18:20). For this reason the faithful should participate in the apostolate by way of united effort. They should be apostles both in their family communities and in their parishes and dioceses, which themselves express the community nature of the apostolate, as well as in the informal groups which they decide to form among themselves. The group apostolate is very important also because the apostolate must often be performed by way of common activity both the Church communities and the various spheres. For the associations established for carrying on the apostolate in common sustain their members, form them for the apostolate, and rightly organize and regulate their apostolic work so that much better results can be expected than if each member were to act on his own. In the present circumstances, it is quite necessary that, in the area of lay activity, the united and organized form of the apostolate be strengthened. In fact, only the pooling of resources is capable of fully achieving all the aims of the modern apostolate and firmly protecting its interests. Here it is important that the apostolate encompass even the common attitudes and social conditions of those for whom it is designed. Otherwise those engaged in the apostolate are often unable to bear up under the pressure of public opinion or of social institutions.
Source: Apostolicam Actuositatem (Vatican.va)