Your CrewMates do not have to be NauticEd students. If they are not a NauticEd student, they will simply get an email asking if the logbook entry you made that included them is a real sailing adventure. They just need to respond affirmatively to the email and your logbook entry is then authenticated.
Becoming a CrewMate of yours does not set them up an account with NauticEd automatically. At any time your CrewMate can decide to become a NauticEd student, simply they should set up an account with NauticEd using the same email address as the CrewMate email address that you used to set them up as your crew mate.
BIG NOTE: Ask your potential CrewMate if they are already a NauticEd Student. If so, confirm their NauticEd User email address first and use that when you set them up as your CrewMate. IF YOU DON'T DO THIS, your entries will not connect with their own already NauticEd account and that will cause headaches for you, your CrewMate and our support team.
If you make an entry into your logbook with a CrewMate present and the ARE a NauticEd student then they can simply login and authenticate the entry - they will also get an email in which can authenticate right from the email.
CrewMates are not required for a logbook entry - however and authenticated entry does give your logbook more credibility. Not that you would do it but don't set up a false account to do authentications - as with life, if you do things like this the karma police will knock you down hard causing you to lose any credibility. Charter companies will likely ban you and nothing good will come of it.