Can other people see Misaki?
Yes. 


   In episode 5, you (the viewer) learn that the class is ignoring Misaki as part of a "countermeasure" to ward off the calamity that affects class 3-3 (which is what causes all the paranormal phenomena you see in episodes 1-4, e.g. the girl nearly getting crushed by the falling glass pane; the nurse in the crashing elevator; etc). The basic idea is that the class has one more person than it should (you find out why in episode 6, I believe), and so by pretending that one of the people in the class doesn't exist, they bring the number of people in the class down to the correct number.