Staff Members are managed at the organization level, so you can add them to groups which in turn gives them access to as many individuals as needed all at once. A good example of this would be a teacher who needs access to her students in her classroom.
Guest on a profile is meant to give access for ONE person to ONE individual profile. A good example of this would be a parent who needs access to their child’s profile which is managed by a school, or a doctor who needs access to see if a new medication is working as expected.