A cooperator is just what the word means. It is someone who, while not being a member of the Work, would like to cooperate with Opus Dei in its apostolic activities. Anyone who believes what Opus Dei is trying to do is worthwhile may be a cooperator, including non-Catholics, non-Christians, and even those who don’t believe in God.
Cooperators agree to help the Work first of all by praying for it every day. Secondly they assist in some material way, such as assisting in one of the Work’s apostolic activities, or making a financial contribution.
In return, members of Opus Dei pray for the cooperators every day in the Preces (See Question 42). They may also partake in special means of formation Opus Dei offers to cooperators, like the circles mentioned in Question 23, or workshops, spiritual direction, doctrinal classes, etc. The church has also granted cooperators the privilege of receiving a plenary indulgence each year on the anniversary of the day they became a cooperator, under the usual conditions.
November being the month when special attention is paid in the church to those who have died, all priests of the Work offer a Mass during November for the repose of the souls of all those cooperators who have died. For one day in November, the members of Opus Dei are asked to offer up the Mass they attend, Holy Communion, and the Rosary for the repose of the souls of the deceased cooperators.
Often people who are not yet ready to join Opus Dei become cooperators instead, but that is not a requirement, and most cooperators never become members.