As an instructor/assessor when you sign in you will see the following. Enter your student's name and logbook code and tap add student.
Note (1) they need to already be a student and (2) you need their logbook code which the student needs to communicate that to you. Make sure you say "Your Logbook code NOT your password. Your logbook code is under your profile area when you signin (top right dropdown by your name)".
If they are not a student you can use www.nauticed.org/quickadd