Peritus
Peritus (Latin for "expert") is the title given to Roman Catholic theologians attending an ecumenical council to give advice. At the Second Vatican Council, some "periti" (the plural form) accompanied individual bishops or groups of bishops from various countries. Others were formally appointed as advisers to the whole council.

At the First Vatican Council, John Henry Newman refused an invitation to be a "peritus".

Joseph Ratzinger who became Pope Benedict XVI served as "peritus" to Cardinal Josef Frings, Archbishop of Cologne, Germany, while Hans Küng was a "peritus" for the Second Vatican Council, rather than for an individual bishop. 

The German theologian Karl Rahner served as "peritus" to Cardinal Franz König of Vienna. 

Cardinal Yves Congar served as a consultant to the council upon the invitation of Pope John XXIII, but was hired as personal and expert theologian (peritus) at the council to Bishop Jean-Julien Weber of Strasbourg which allowed him to attend all the general sessions and to participate in discussions of any commission to which he was invited to.

The "periti" of Vatican II, although their official status designates them as at the service of the council, were most often first of all at the service of the currents which clashed at the council, either on the side of a reform of the church (acceptance of religious freedom, revision of church–state relations, reassessment of relations with other religions), either on the side of the continuation of a clash of Catholicism with the other institutions and currents of society, in the wake of the councils of Trent and Vatican I (especially for the experts close to Coetus Internationalis Patrum).

As for the US, there were 86 periti altogether during the four sessions of the council.
Q397
Latin
Concept
Q51645
ecumenical council
Concept
Q169789
Second Vatican Council
Concept
Q190857
First Vatican Council
Concept
Q44490
John Henry Newman
PER
Q2494
Joseph Ratzinger
PER
Q45722
Cardinal
Concept
Q44589
Josef Frings
PER
Q123010
Hans Küng
PER
Q169789
Second Vatican Council
Concept
Q57317
Karl Rahner
PER
Q78546
Franz König
PER
Q363970
Yves Congar
PER
Q23873
Pope John XXIII
PER
Q476451
Peritus
MEDIA
+86^^http://www.w3.org/2001/XMLSchema#decimal
86
NUMBER
participant
position held
participant
participant