As already pointed out the student ID is the logical username. In certain cases it makes sense to have the full e-mail address, studentID@example.com, as the username too, assuming that your students get mail boxes of that format.
The communication for the initial log in can be done through one of the methods documented in https://docs.moodle.org/en/Upload_users. The most common one is not to have an email column at all. Then Moodle will send an invitation.