There are several things you can do if you need more space on your Dropbox account.
How to earn free space
Note: Sharing a folder doesn't count as a referral.
Upgrade your account
If you have a Dropbox Basic account, you can upgrade to Dropbox Plus to get 2 TB of space or Dropbox Professional to get 3 TB of space. If you have a Plus or Professional account and work with two or more colleagues, you may want to consider upgrading to Dropbox Business, which comes with 5 TB of space.