Contacts are those individuals you've shared a file or project with individually. For instance, your friend or maybe someone you're trying to entice to join your project.
Contacts maintain their own Studios and your files are shared with them just when you select to add them to the file.
Studio Members are people with their own accounts but who share a studio with you. This is great for in-house projects, centralized billing, and workplaces.
By default, projects are shared with Studio Members. You can enable or disable sharing of projects and/or individual files to either your entire studio or to individual studio members or contacts.
Here's a quick video on inviting members and contacts to your Studio.