I think there are three possibilites.
1. It is used as the royal 'we'.
2. God is talking about the heavenly host of angels.
3. God is speaking about the other gods in the Canaanite pantheon.
Depending on which stance you wish to take, you could argue for any.
I think Christians will adopt 1 or 2, as 3 negates their monotheism.
Studying the Hebrew Bible objectively, I think option 3 is the most likely, as polytheism is heavily supported in the early books of the Hebrew Bible.