From the 2002 General Instruction of the Roman Missal (GIRM) approved for the USA, which can be accessed from
romanrite.com/girm.html :
“138. … After the intentions, the priest, with hands extended, concludes the petitions with a prayer.
The Liturgy of the Eucharist
139. When the Prayer of the Faithful is completed, all sit, and the Offertory chant begins (cf.
above, no. 74).”
“74. The procession bringing the gifts is accompanied by the Offertory chant (cf. above, no.
37b), which continues at least until the gifts have been placed on the altar. The norms on the
manner of singing are the same as for the Entrance chant (cf. above, no. 48). Singing may always
accompany the rite at the offertory, even when there is no procession with the gifts.”
“48. The singing at this time is done either alternately by the choir and the people or in a
similar way by the cantor and the people, or entirely by the people, or by the choir alone. In the
dioceses of the United States of America there are four options for the Entrance Chant: (1) the
antiphon from the Roman Missal or the Psalm from the Roman Gradual as set to music there or
in another musical setting; (2) the seasonal antiphon and Psalm of the Simple Gradual; (3) a song
from another collection of psalms and antiphons, approved by the Conference of Bishops or the
Diocesan Bishop, including psalms arranged in responsorial or metrical forms; (4) a suitable
liturgical song similarly approved by the Conference of Bishops or the Diocesan Bishop.55 …”.
Reciting a prayer together is not singing something together, so the GIRM is not being followed.