When God said, “Let us make man in our image” who was God speaking to?

God was speaking to the angels.

"And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth." - Genesis 1:26

"So God created man in his own image, in the image of God created he him; male and female created he them." - Genesis 1:27

Where does it say in the Bible that God and the angels make appearances in the form of man?

The Bible tells us that the LORD appeared to Abraham along with two angels. They appeared to Abraham as three men.

"And the LORD appeared unto him in the plains of Mamre: and he sat in the tent door in the heat of the day; And he lift up his eyes and looked, and, lo, three men stood by him: and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground," - Genesis 18:1-2

You can read why they appeared to Abraham and the conversation they had in the rest of chapter 18. After a certain point, the two angels left to go to Sodom.

"And the men rose up from thence, and looked toward Sodom: and Abraham went with them to bring them on the way." - Genesis 18:16

The third man remained to finish talking with Abraham. Who was this third man?

"And the men turned their faces from thence, and went toward Sodom: but Abraham stood yet before the LORD." - Genesis 18:22

The Bible is telling us in this verse, that Abraham stood yet before the LORD. The LORD appeared in the form of man to Abraham.

Where in the Bible does it say that the other two men that left Abraham and went to Sodom were angels?

It tells us this in the first verse of the next chapter:

"And there came two angels to Sodom at even; and Lot sat in the gate of Sodom: and Lot seeing them rose up to meet them; and he bowed himself with his face toward the ground;" - Genesis 19:1

Because angels appear to us in the image of man, we can understand this verse:

"Be not forgetful to entertain strangers: for thereby some have entertained angels unawares." - Hebrews 13:2

Abraham showed hospitality to angels without being aware of it. Lot also entertained angels unaware. Man was made in the image of God and of angels. Abraham and Lot entertained angels unawares because the angels appeared to them as men. After Abraham entertained the two angels, Lot entertained them when they came to Sodom.

"And he said, Behold now, my lords, turn in, I pray you, into your servant's house, and tarry all night, and wash your feet, and ye shall rise up early, and go on your ways. And they said, Nay; but we will abide in the street all night. And he pressed upon them greatly; and they turned in unto him, and entered into his house; and he made them a feast, and did bake unleavened bread, and they did eat." - Genesis 19:2-3

We too may have entertained angels unawares.