Attached are a few screenshots that should help. The Trip Alert Registration form is what the traveler will need to complete after they click on the "Register Here" link from their Opt-in invitation.
The traveler will then need to complete their Trip Alert profile to setup the "Pre-Trip" and "Traveling" contact points once they've completed the first step. Both Pre-Trip and Traveling contact points are required if a traveler has opted-in to receive messaging. However, if they wish to disable only Pre-Trip alerts, the email address can be updated with a false one (such as firstname.lastname@example.org) to prevent those alerts from being sent to their valid email address.
To resend an Opt-in invitation, please see these FAQ instructions: