When you sign in and click on the logbook function, you will see a crewmates link (circled in red here).
You'll also see crewmates that have not accepted to be crewmates yet. You can resend them the email by tapping the resent button.
BUT wait - chances are the email address you have for them is old or not checked often or it had a fat finger applied to it. Also, make sure that the email you have for them is their NauticEd email (if they are already a student). This is important so that the logbook entries goes to their own NauticEd logbook. No biggy if they don't have an account.
If your CrewMate has an account with NauticEd, only one of you needs to make the entry, then when the other authenticates, the entry is auto-added to their logbook.
We only send one authenticate email per week. It will contain all the entries made that week. We do this so that it respects your friends inbox. They can then tap "Authenticate A'', if there is more than one entry.