First, be sure that you invited your students to the private access group specifically, and not just to the Discover More Corps site in general. Second, your students must use the same email address to join the Discover More Corps that you used to invite them. For example, if you sent the invitation to joe.student@school.edu, but your student registered for the Discover More Corps using his personal email, such as jstudent@gmail.com, he will not be able to access the private group.