In Act I, Scene 2, Horatio comes and tells Hamlet that the ghost of his father has been appearing. In the previous scene, Horatio and three other men (Barnardo, Marecellus and Francisco) are on guard duty. The last of these three have seen the ghost before so they have asked Horatio to come and keep watch with them to confirm what they saw. The ghost comes again and all four see him.
In Act I, Scene 2, all four of them come to see Hamlet. After talking to him a bit, Horatio tells him that the ghost has appeared.