NOTE: Any files you want to share MUST be in your Google Drive Folder!
Right click on the file you want to share, and in the context menu, move the cursor to "Google Drive" and then select "Share..."
In the invite people box at the bottom of the menu, type the name or email of the person(s) you are trying to share the document with. To the right of the invite box, select whether you want to give people view or edit access. Once your done click the send button.
Opening files shared with you
Click on the Google Drive icon in the menubar at the top of your screen.
Select "View items shared with me"
Log in using your Google Drive Credentials (See the Google Drive Setup tutorial for more information).
The files that have been shared with you are listed. Please note, these files are not synced to your Drive, to do so, you must select the checkbox next to each file and click "Move to Drive" at the top of the list.