Are there any websites like this?

[Carolyn]

I need a website in which I can share documents/notes with 2 of my friends, and they would have the ability to add to the notes or modify them?

Better yet, everyone else can see the changes that that person has made?

That's all, really. Thanks so much!

[good question, sir]

https://www.blogger.com/home

make a private blog and invite your friends to be authors.

[Joseph]

There's this: https://docs.google.com
This is software, but I'm pretty sure that this works on the cloud: https://evernote.com/evernote/

[sasari]

Hi. Considering your requirements, I think Zoter (http://www.zotero.org/) is the best solution. It is a reference tool. But it allows collaborative works and can have library online and share with others with different access levels.