You can get extra space by inviting your friends to try out Dropbox. If a friend uses your invitation to sign up for an account, installs the Dropbox desktop app on a computer, and signs in to the app, both of you will receive bonus space.
You can track the status of your referrals from the bonus space tab of your account settings.
Note: Your friend needs to install the Dropbox desktop app on a computer and sign in to the app in order for you to get the extra space. Installing only the Dropbox mobile app on a phone or tablet will not qualify.