In Act I Scene 1, what did Horatio and his group decide to do after they had seen the ghost?
The guards had invited Horatio to see the ghost because he was a "scholar," Marcellus says, and perhaps also because he is Hamlet's friend. Having seen the ghost and ascertained that he is not a figment of their imagination, and further that he is the spirit of the now-deceased King Hamlet, Horatio and his companions decide to tell Hamlet what they have seen:
Let us impart what we have seen tonight
Unto young Hamlet; for, upon my life,
This spirit, dumb to us, will speak to him.
Interestingly, Horatio assumes that the ghost has appeared to the men because of the impending conflict against young Fortinbras of Norway. When Hamlet meets the apparition in scenes four and five, the audience discovers that this is not at all the reason the dead king's ghost haunts the castle.