Each 2.009 team has a Dropbox folder provided through MIT's Dropbox for Business account. You will be invited to your team's dropbox folder via your MIT Athena username email address.
To access your team's Dropbox folder, you will need to register to use MIT's Dropbox for Business account. To do this, visit https://dropbox.mit.edu/ to sign up for an account. Then, you will receive an email invitation to complete the registration process. You must be a current MIT student, faculty member, or staff member to receive an account through this process. Email Professor Wallace with your Athena username if the above web page will not let you register for an account, so you need 2.009 to request a Dropbox account for you (e.g., you are an affiliate).
If you already have a personal Dropbox account associated with your MIT email address, you will need to choose whether you want to associate your personal account with a non-MIT email address instead or to move all the files into your MIT Dropbox account as described in the KB article. Dropbox permits you to "link" your non-MIT email address personal account with your MIT Dropbox account, so that you can easily switch between the two accounts in the Dropbox interface.
MIT's Dropbox for Business account has enabled "single sign-on", which means that you login to access Dropbox using your MIT credentials (certificate or Athena username/password) and duo. Once you login, you will need to accept the invitation to the shared team folder by going to the "Sharing" area of the website. Once you have accepted the invitation to join the team folder, it will show up in your "Files" list. The benefit of using the MIT account for your 2.009 files is that the files will not count against your personal quota.
Note: Because of the single sign-on process, if you have MIT certificates installed on a web browser, you will only be able to access the MIT Dropbox account associated with the MIT certificate using that web browser until that authentication session expires in 8 hours. If you need to access a different MIT Dropbox account, you will need to explicitly log out of Dropbox and close the web browser.