The size of a shared folder will be counted against the quota of every member of that folder, unless everyone is a member of the same Dropbox for Business account. For Business accounts, the size of the shared folder will be counted against the team's shared quota only once.
For all other users, your Dropbox storage quota is calculated by adding up the total amount of data in your Dropbox folder, including all shared folders. As a result, the owner of a Dropbox Free 18 GB (2 GB + bonus storage earned through referrals) account will not be able to collaborate on a shared folder larger than 18 GB. One reason this policy exists is to prevent people from giving themselves unlimited space by stacking free accounts with shared folders.
This article refers to the Dropbox for Business product. Dropbox for Business is designed for projects, groups, and other organizations that want to share the same Dropbox storage quota with all of its members. In this instance, if you share a folder with someone else on your team, the shared folder uses only the team's aggregate storage quota. However, if you share a folder with someone not included in your team, the behavior above applies; shared folders will use up both your team's quota and your external colleague's storage quota.
If you're interested in learning more about Dropbox for Business or want to sign up, visit the Dropbox for Business webpage.