A significant part of Christian piety in the late-Middle Ages were the various guilds and confraternities associated with different saints. These confraternities were made up of groups of parishioners who supported each other in their devotion to a particular saint. Members would gather for prayers, furnish an altar or decorations for an image of the saint, and pray for the intentions of all the members of the guild. The benefits of joining a confraternity extended even after death. Members would promise to pray for the deceased members of their confraternity and would endow a series of Masses to be said for the souls of any member who died. These groups were not limited only to the rich or the learned. Pious confraternities were made up of both wealthy and poor, educated craftsmen and peasant farmers. Guilds formed the glue of parochial life and helped to connect the members of local society through their devotions. 

One such group that came about in the middle of the fifteenth century was the Confraternity of the Rosary. Inspired by the preaching of the Dominican Alan de la Roche, the first confraternity dedicated to the Rosary was erected in Cologne in 1447. Soon, many similar confraternities were established and they were placed under the care of the Dominican Order. These confraternities are still active today with countless members around the world. Many Dominican saints and blesseds, especially the martyrs of South East Asia, were active in promoting and participating in their local confraternities. 

The Confraternity of the Holy Rosary is one of the most highly indulgenced pious associations of the faithful. Members of the Confraternity (called Rosarians) through the generosity of the Church, are offered great spiritual advantages including sharing in all the Masses, Divine Offices, and good works of the fathers, brothers, contemplative nuns, sisters, and laity of the whole Dominican Order. Additionally, plenary indulgences are made available to Confraternity members on the day of their enrolment, Christmas Day, Easter Sunday, the Assumption, the feast of Our Lady of the Rosary, and the Immaculate Conception. A plenary indulgence is also available daily for those members who recite five decades of the Rosary at once. 

Confraternity chapters at parishes will often pray the Rosary in common, but even when a Rosarian prays the Rosary alone he is united to all the members of the Confraternity praying the Rosary on behalf of the Church, in virtue of their membership in the public association. Rosarians, when they pray the Rosary, pray for the intentions of the whole Confraternity, tapping into all the graces offered by the Church to the Confraternity and its members.

Surely all the lavish benefits of enrollment in the Confraternity must require some great labor  or special standing on the part of the members of the Rosary Confraternity. But no. Members can enroll online or through their local parish chapter, and, once enrolled, the only requirement is that members pray fifteen decades of the Rosary each week and include the other members and their intentions in their prayers—such great spiritual benefits for something many members would do even without the Confraternity.  
You can learn more about the Confraternity of the Rosary here.