First you need this set of relationships:
Courses::__pkCourseID = Roster::_fkCourseID
Students::__pkStudentID = Roster::_fkStudentID
Your portal would refer to Roster, not Students so that you can enroll a given student in multiple courses. You would indeed have multiple records for the same student, but in Roster, not Students.
For an explanation of the notation that I used, see the first post of: Common Forum Relationship and Field Notations Explained
This, BTW, is called a many to many relationship. Here's a demo file on many to many relationships that you may find to be helpful: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7