There is no limit to the number of students who can join a course.

On the home page for a course, the instructor for a course can find the code for the course. The instructor can Give this code to students in their course and the students can join your course using this code.

Students need the course code to join a course. They can get this code from the instructor. The code can be used either when a student is joining Courseville or if the student is already a member of Courseville, then by clicking on "Join a Course..." link in left sidebar.