Yes. You need a Dropbox account to join a shared folder. If you received a shared read-only link to a file or folder, then you don't need to have an account. But if you have received an invitation to a shared folder, then you need to have a Dropbox account to join it. The reason behind this is, the files in a shared folder are synced between members. whereas links which are not synced that allow recipients to view and download files and folders on their computer. If you don't have a Dropbox account registered to the email address where you received the invitation, then you will need to create an Dropbox account with that email address.