In act I is the ghost is seen Marcellus, Bernardo, Horatio and Hamlet?
This is true. The sentry see the ghost of King Hamlet for many nights and then decide to tell young Hamlet about the appearance. The ghost leads Hamlet away from the others and speaks with him about how he was murdered and who did it, which puts the rest of the plot into perspective. Hamlet spends the rest of the play deciding what to do about the information he receives from the ghost of his father.