No, at the end of the priest’s invitation.
The priest says: “Pray, brethren, that our sacrifice may be acceptable to God, the almighty Father.”
The people stand and respond: “May the Lord accept the sacrifice at your hands …”.
The 2002 General Instruction of the Roman Missal (GIRM) is a little unclear:
“43. The faithful should stand … from the invitation,
Orate, fratres (Pray, brethren), before the prayer over the offerings …”.
The Order of Mass, in the 2002 Roman Missal is more specific. It has the priest’s words, then the rubric:
“Populus surgit et respondet”.
This means the people stand and respond.
Then it has the words the people say. (From Missale Romanum, Libreria Editrice Vaticana, 2002, ISBN: 8820972719, page 515).
People need some warning before they stand together. At the beginning of the priest’s invitation they have no warning, so it would not work.
What if incense is used? Should the people stand when they are incensed? Should they remain standing after it, while the priest says the invitation, “Pray, brethren”.
Generally the liturgical books give no direction for them to stand at different times if incense is used. The exception is in the Ceremonial of Bishops (Caeremoniale Episcoporum), in describing the Stational Mass of the Diocesan Bishop, n. 149: “… The bishop receives the censer from the deacon and, in the same way as at the beginning of Mass and accompanied by a deacon, incenses the gifts, as well as the altar and the cross. After this,
all rise, and a deacon, standing at the side of the altar, incenses the bishop, who stands without the miter, then the concelebrants, then the people.” There are no directions for the people to sit. So a unique feature of the Stational Mass is that all stand when the thurible is passed from the bishop to the deacon.
From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“112. … At a Mass celebrated by the Bishop or at which he presides without celebrating the Eucharist, the norms found in the
Caeremoniale Episcoporum should be observed.”
Reference: Ceremonial of Bishops, Liturgical Press, 1989, ISBN 0-8146-1818-9, page 56.