It seems demons can inhabit physical beings like the pigs in this passage (Matthew 8:30-34):
30Some distance from them a large herd of pigs was feeding. 31The demons begged Jesus, "If you drive us out, send us into the herd of pigs." 32He said to them, "Go!" So they came out and went into the pigs, and the whole herd rushed down the steep bank into the lake and died in the water. 33Those tending the pigs ran off, went into the town and reported all this, including what had happened to the demon-possessed men. 34Then the whole town went out to meet Jesus. And when they saw him, they pleaded with him to leave their region.
And Satan can enter into humans as well as in this passage (John 13:25-27)
25 Then, leaning back[d] on Jesus’ breast, he said to Him, “Lord, who is it?”
26 Jesus answered, “It is he to whom I shall give a piece of bread when I have dipped it.” And having dipped the bread, He gave it to Judas Iscariot, the son of Simon. 27 Now after the piece of bread, Satan entered him. Then Jesus said to him, “What you do, do quickly.”
Thus, spiritual beings such as demons and Satan seem not to be omnipresent since it wouldn't make sense to say a demon entered into a person if they're everywhere. However, this might not be the whole story because God is known to be Omnipresent but is not IN everyone yet. However, He definitely is, was, and will be in Jesus. So, I'd like to hear what Ray has to say about this.