When you upload files to the "My Drive" portion of Google Drive, you are the owner by default and only you have access to the files. If you want to share these files with collaborators either on or off campus, choose "Share.." on the file or folder.
At the bottom right of the "Share with others" window, click Advanced.
On the next screen, add the email addresses of your collaborators. Swarthmore addresses will auto-complete as you type them, external email addresses will not. You can give each collaborator edit permissions to the file or View only permissions as you add them. If the "Notify people" box is checked, a default Google email with a link to the file or folder will be sent automatically. If you un-check the box, you can send the link manually (more on this below).
The best way to ensure confidentiality on a file is to only grant permissions to your direct collaborators. Sometimes you may need a document to be accessible to more people, however. To make a document more publicly accessible, choose "Change.." next to "Specific people can access".
This will bring up the "Link Sharing" options. Setting these options will allow a link to your file to be forwarded via email or linked to a web page. These settings should be used for public information only. Read the sharing descriptions for specific details.
Last but not least, if you want to send out personal links to files stored in Google Drive instead of the automated notifications, you just need to copy and paste a link to the file into an email. Copy and paste the "Link to share" URL from the top of your Sharing Settings window into your notification email. Only recipients who have already been granted access to the file or folder will be able to access the link.
Google Drive Folder Sharing Notifications
Google Drive allows users to email collaborators from a specific file but not for a folder. Folders only allow you to send notifications on new shares, and doesn't provide an easy way to export an existing permissions list, especially if the collaborator list is long.
Within the shared Folder in question, create a Google Doc file with the same sharing permissions as the containing Folder. The file should inherit permissions of the folder automatically.
Name the Google Doc something helpful like "Folder Collaborator Notice."
From the containing Folder properties (right-click menu, "Get link"), get the "share link" and paste it into the Doc (yields a clean look to make this link the only line in the Doc).
Within the Google Doc, pick "File/Email collaborators"
Edit the Send Message dialogue as needed then send. (For the clean look mentioned in 3, pick "Paste the item itself into the email") You probably want to check the box to send a copy to yourself as well.
Save the Google Doc (in that containing Folder) for future and/or repeated use.