We use Microsoft SharePoint for securely sharing files.  Here's how it works:

1) You'll receive an email with a link to your shared folder.

2) Click "Open" and sign in with your email address.  No password is required since you'll be provided with a "Verification Code" in the next step.

3) Check your email for a "Verification Code".  Enter this code and click "Verify".

4) You will now be signed in and will have access to your shared files and folders.