The Guest Student role is what administrators use when creating guest accounts for persons who do not have Hawk IDs. Students and Guest Students have nearly identical rights in ICON course sites. The only difference from the user's perspective is that those with Guest Student status can change their passwords within ICON while University-affliliated (Hawk ID) users must use the password management tools in ISIS.
For more information on roles and rights of students, see the online help section ICON Roles & Rights for Students / TA's / Instructors-Designers.