The easiest way is to give your student a sign-up code and have them join a class themselves. Instructions for that are here.
If you prefer to add students yourself, though, sign-in and navigate to your Admin screen. Click on the 'Students' tab, next to your 'Classes' tab.
Then, select 'Add Students' and enter the emails of the students you wish to add to your class, separated by commas and add them to the class.
If the student has an account on Veritas, using that email address, it will email them a link which they can use to be added to the class, and notify them of a pending invitation when they next log in to the site. If that email does not appear in the Veritas system as a student, it will email the student with an invitation to create an account.
Note: Students are never automatically added to a class, and MUST accept the invitation either through the email link or by logging into the site.
You will be able to view the invitations which you have sent to add students to class from your Admin panel. From here, you can view all unaccepted invites: the email addresses, when the invite was sent, and also re-send or delete the invitation.