HP7 question about Slytherin's loyalty?

When Lucius asked Voldemort about finding Draco, Voldemort asked Lucius why Draco had not come out to join him like the other Slytherins; perhaps he became friends with Harry Potter. Did the Slytherins join Voldemort? How could they have if they were evacuated to the Hog's Head? Also, whose side were the spiders on?