Answer:
Hamlet
Barnardo
Marcellus
(Horatio - you weren't asked this but you should know it).
Explanation:
The Ghost appears a total of 4 times. In the beginning, only Barnardo and Marcellus and Horatio see him. The first two are 2 guards. Horatio is the young Hamlet's good friend and confident. The guards ask Horatio to speak for them, since he is well educated and best able to deal with whatever the ghost has to say. The Ghost is about to answer Horatio's questions when dawn breaks and he (The Ghost) disappears.
Horatio persuades Hamlet to stay up late and hear what the Ghost has to say. The Ghost says
"...but know, thou noble youth,
The serpent that did sting thy Fathers life,
Now wears his crown."
Those are the people that see the ghost. He talks mostly to Hamlet, but Hamlet also sees him.
