Objection 1. It would seem that the angels who are sent also assist. For Gregory says (Hom. xxxiv in Evang.): "So the angels are sent, and assist; for, though the angelic spirit is limited, yet the supreme Spirit, God, is not limited."
Objection 2. Further, the angel was sent to administer to Tobias. Yet he said, "I am the angel Raphael, one of the seven who stand before the Lord" (Tobit 12:15). Therefore the angels who are sent, assist.
Objection 3. Further, every holy angel is nearer to God than Satan is. Yet Satan assisted God, according to Job 1:6: "When the sons of God came to stand before the Lord, Satan also was present among them." Therefore much more do the angels, who are sent to minister, assist.
Objection 4. Further, if the inferior angels do not assist, the reason is because they receive the Divine enlightenment, not immediately, but through the superior angels. But every angel receives the Divine enlightenment from a superior, except the one who is highest of all. Therefore only the highest angel would assist; which is contrary to the text of Daniel 7:10: "Ten thousand times a hundred thousand stood before Him." Therefore the angels who are sent also assist.
On the contrary, Gregory says, on Job 25:3: "Is there any numbering of His soldiers?" (Moral. xvii): "Those powers assist, who do not go forth as messengers to men." Therefore those who are sent in ministry do not assist.
I answer that, The angels are spoken of as "assisting" and "administering," after the likeness of those who attend upon a king; some of whom ever wait upon him, and hear his commands immediately; while others there are to whom the royal commands are conveyed by those who are in attendance--for instance, those who are placed at the head of the administration of various cities; these are said to administer, not to assist.
We must therefore observe that all the angels gaze upon the Divine Essence immediately; in regard to which all, even those who minister, are said to assist. Hence Gregory says (Moral. ii) that "those who are sent on the external ministry of our salvation can always assist and see the face of the Father." Yet not all the angels can perceive the secrets of the Divine mysteries in the clearness itself of the Divine Essence; but only the superior angels who announce them to the inferior: and in that respect only the superior angels belonging to the highest hierarchy are said to assist, whose special prerogative it is to be enlightened immediately by God.
From this may be deduced the reply to the first and second objections, which are based on the first mode of assisting.
Reply to Objection 3. Satan is not described as having assisted, but as present among the assistants; for, as Gregory says (Moral. ii), "though he has lost beatitude, still he has retained a nature like to the angels."
Reply to Objection 4. All the assistants see some things immediately in the glory of the Divine Essence; and so it may be said that it is the prerogative of the whole of the highest hierarchy to be immediately enlightened by God; while the higher ones among them see more than is seen by the inferior; some of whom enlighten others: as also among those who assist the king, one knows more of the king's secrets than another.
Source: Summa Theologica (New Advent)