Wikipedia describes the Priestly Society of St. Peter this way:
<The Priestly Society of the Holy Cross is an association of Catholic diocesan priests which is integrally united to the Prelature of Opus Dei. Part of the society is made up of the clergy of the Opus Dei prelature—members of the priesthood who fall under the jurisdiction of the Opus Dei prelature are automatically members of the Priestly Society. Other members in the society are traditional diocesan priests--- clergy who remain under the jurisdiction of their diocesan bishop. Technically speaking, such diocesan priests have not "joined" Opus Dei membership, although they have joined a society that is closely affiliated with Opus Dei.>
I infer that Finn is still a priest of the diocese of Kansas City and subject to the bishop. He is not incardinated to Opus Dei and not technically a member. However, as a member of a society that is "integrally related" to Opus Dei, it seems to be a distinction with no practical difference.