What common knowledge is there between the men at the opening of the scene? How does each singularly?

The guards know about the appearance of the ghost of Hamlet Sr. They share the knowledge with Horatio when then knows about the apperance of the ghost. Once Horatio sees the ghost for himself, he understands that the ghost will not speak to anyone except Hamlet Jr. so he and the guards make a move to let Hamlet Jr. know what is going on.