Almost as soon as we were born, our parents told us we were supposed to share. Google makes that easy. You can share almost anything. You can share documents, calendars, blogs, and even share editing for an entire website. Sharing documents among students opens up a whole new world of possibilities. Think class group work. Think organizations. Think sharing with your professor for comments or a virtual office visit. Once a document is shared, all members can edit it at the same time. You can also turn chat on and suggest information back and forth through that medium. I'll do some screenshots below, but you can also view a quick video and watch it in action on the HackCollege blog where he shares his ideas for collaborating with your class friends by taking notes together on the same document.
Create any type of document - document, spreadsheet, presentation, etc - by clicking on Create and choosing from the drop down menu.
Within the document, click on the Share button in the top right.
Click on the text box below "Add people" and type in an email address. (Note that you can also share the document with a Google group. Organizations, are you getting ideas yet?) To the right, you can choose if the person can edit, make comments, or just view. Then you can choose to let the person receive an email that the document has been shared with them and even add your own message. Click "Share & save" and you're done. Let the group edits begin.