You can be a member of Hogwarts on Pottermore.com
No, education at Hogwarts ends after the seventh year.
Hermione Granger
They are called roleplaying games and there are hundreds of them out there. Some are different than others though; some are based on forums other are based on websites.
myHogwarts.com Hexrpg.com EnchantedHogwarts.com LumosAlliance.com
Ollivander
Wear a pointy witch hat and long dress robes.
Cederic Digory
Why yes He is, based on the results of his OWLs .
As a student at Hogwarts, Arthur was in Gryffindor.
Poliakoff is the surname of a Durmstrang student who came to Hogwarts during the Triwizard Tournament.
Ernie McMilan is a Hufflepuff student at Hogwarts school and is also a member of Dumbledore's Army.
Michael Corner is a Ravenclaw student at Hogwarts school and is also a member of Dumbledore's Army.