Theophylact of Ohrid

And "He calls together His friends and neighbors," perhaps the Angels, whom we also understood as the sheep, in a twofold sense. Since, on the one hand, every created being in relation to God is, as it were, without reason, the Heavenly Powers can be called sheep. But since, on the other hand, they are rational, that is, endowed with reason, and appear to be nearer to God than the rest of creation, the ranks of the angelic Powers can be understood as friends and neighbors.

Commentary on Luke ↗

Gregory the Dialogist

But having found the sheep, he returns home; for our Shepherd having restored man, returns to his heavenly kingdom. And hence it follows, And coming he collects together his friends and neighbours, saying to them, Rejoice with me, for I have found my sheep which was lost. (1 Pet. 2:24, Isai. …

Catena Aurea by Aquinas ↗

Bede

And coming home, he calls together his friends and neighbors, saying to them, Rejoice with me, for I have found my sheep that was lost. Having found the sheep, he returned home, because our Shepherd returned to the heavenly kingdom after restoring man. There he found friends and neighbors, namely those choirs of angels. Who are his friends, because they continually keep his will in their stability. They are also his neighbors, because they enjoy the brightness of his vision with their constant presence. …

On the Gospel of Luke ↗

Bonaventure

Secondly, with regard to the public rejoicing, he adds: And coming home, he calls together his friends and neighbors. This house is the heavenly Jerusalem, of which Isaiah 60: "I will glorify the house of my majesty"; and John 14: "In my Father's house there are many mansions." Into this house Christ came at the ascension, concerning which coming Isaiah 63: "Who is this who comes from Edom, with dyed garments from Bozrah?" Into this he carries the sheep, because, as is said in Ephesians 4, "ascending on high, he led captivity …

Commentary on Luke, Chapter 15 ↗