Wait... Wait... Wait...
I am very confused... Does it mean:

a) They give you a part and you need to add on...
B) You make the whole fanfic...
c) One person starts and another ads on ect...

I am very confused.... Writing is what I am best at... Try reading some of my other fics...

The Missing Weasley - http://hpfanfiction.nexcess.net/storyline/...ory.php?no=3414
Harry Potter and The Deck of Choice - http://hpfanfiction.nexcess.net/storyline/...ory.php?no=2104 *This one is more advanced for my 14 years of age... hee hee*