Here is the oration that the Church prays in the Mass and Office for the Holy Guardian Angels (October 2):
Deus, qui ineffábili providéntia sanctos Angelos tuos ad nostram custódiam míttere dignáris: largíre supplícibus tuis; et eórum semper protectióne deféndi, et ætérna societáte gaudére.
Here is my translation:
O God who by Thine ineffable providence didst deign to send Thine holy Angels for our protection: grant to Thy suppliants both to be always defended by their protection, and to rejoice in their eternal fellowship.
Here is the translation from the Divinum Officium site:
O God, Who in thine unspeakable Providence hast been pleased to give thine holy Angels charge over us, to keep us, mercifully grant unto our prayers, that we be both ever fenced by their wardship here, and everlastingly blessed by their fellowship hereafter.
The idea that God has deigned to send (míttere) these blessed spirits to guard us is built right into their very name: “Guardian Angels” (angelī cū̆stōdēs). Angelus in Latin comes from the Greek, ἄγγελος, meaning “messenger,” or “one who announces.” Messengers are “sent” by their very nature. A cū̆stōs is a “guard,” “protector,” or “watchman.”
We beseech God in this prayer both to be defended by the protection of our angel guardians and to rejoice in their fellowship in Heaven. All of the Holy Angels — including the guardian Angels — are fellow members with us in the Mystical Body of Christ. For that reason, we enjoy communion with them now by virtue of “the communion of saints,” of which the members of the Church militant, suffering, and triumphant are all a part; but in the Church triumphant, that communion is intensified and made everlasting.