If somebody shared a folder with you in Dropbox, you’re actually notified in either or both ways, i.e., via e-mail or directly to your Dropbox account. On the first end, whether you have a Dropbox account or not, you will still receive the invitation via e-mail because that’s how the sender would and should address you by.

If you already have a Dropbox account, opening your account online will immediately let you know that there is a pending request.

If you received an invitation via e-mail, you may click directly on the link provided (usually, it’s a button with a title View folder).

If you already have an account and is currently signed in, clicking the link will directly open the invitation to you and awaits your consent. Click on Accept or Decline to consider the invitation, or you may click cancel to ignore this request.

If you have an account but are not yet signed it, you will be asked to sign in first.

If you don’t have an account yet, you will be asked to sign up first. Unlike sending URLs or direct links to files which are read-only, sharing folder actually allows real-time file modification from within the Dropbox, hence an account is required to gain access to the invitation or to the folder in general.