What is the role of the Three Persons of the Godhead in creation? Genesis 1:26, New International Version (NIV) reads: 


  Then God said, “Let us make mankind in our image, in our likeness, so that they may rule over the fish in the sea and the birds in the sky, over the livestock and all the wild animals, and over all the creatures that move along the ground.”


This is the first instance in Genesis in which  God addresses Himself as "us". Was God the Father addressing both the Holy Spirit and the Son, or was He addressing only the Holy Spirit of whom  mention is made at Genesis  1:2?
The first thing to keep in mind is that in the Old Testament, doctrine of the Holy Trinity is not at all explicit. The passage in Genesis 1:26 is frequently interpreted as a prefiguration of the Holy Trinity, but it would be a mistake to say that the human author of Genesis is affirming anything about the Divine Persons.

Similarly, it would be a mistake to think that the author had the Holy Spirit in mind when he wrote of the “Spirit of God” (note that the word for “spirit” could also mean “wind”) who “was hovering over the face of the waters” (Gen. 1:2).

In fact, God did not reveal the Holy Trinity to us until He sent Jesus Christ, the Son. It is the “sending”—the “mission”—of the Persons that reveals them to us. (See John 17:18, John 20:21, and 14:16, regarding the Missions of the Persons.)

Naturally, creation is, in fact, the work of all three Persons of the Trinity, and so even though the sacred author knew nothing about the Trinity, the Trinity was at work in the world, from the moment of Creation. God chose to reveal His Triune nature, however, only with Jesus Christ.

Hence, it is possible that the sacred author, under the inspiration of the Holy Spirit, gave the Old Testament a foretaste of the revelation to come by portraying God as refering to Himself as “we.” The “Spirit of God” may indeed be a hint regarding the Holy Spirit. However, from the Old Testament alone, we cannot draw any definitive conclusions about the Holy Trinity—for that we need the revelation given to us by Jesus in the New Testament.

More à propos to the O.P., there is no way to tell for sure if the one “speaking” when God speaks is the Father, or who exactly He is referring to when He says “we.” Certainly, the sacred author did not have the Trinity in mind, but the passage can legitimtely be interpreted in that vein, in the light of the New Testament.