There are three ways for an athlete to see their lineup for a practice: on their dashboard, All Practices page, or via a lineup info email (if the organization allows sending them).
Dashboard
When lineups for a future practice are set, a widget will show up on the athlete's dashboard My Profile -> Dashboard showing their next practice with a button to see their lineup as well as a table showing their next new practices they are attending.
All Practices page
Athletes can see all future (and past) practices in the organization as well as their own lineup for a given practice (assuming they have been placed in a lineup and the lineups have been published) by going to Practices -> Practices and selecting a practice.
Lineup Email
If the organization has enabled sending lineup emails Org Settings -> General, each athlete will receive an email with their boat and seat assignment after lineups for the practice have been published. If their boat or seat changes after an email is sent, they will be sent a new email with their updated seat. Athletes have the ability to turn this email on or off by updating their contact preferences on the My Profile -> Preferences page under the Contact Preferences tab.