2017 Recruiting In the late Biggs D-1AA era, "preferred walk-ons" came in for Fall camp or just after Fall camp, as SloStang mentioned. They got Athletic Dept assistance with admissions/financial aid/class schedules and were treated like regular players in every way. Many earned scholarships and are names you'd recognize.
There were also 10-15 tryout players that enrolled as regular students in Fall (without any team association), joined workouts in Winter, and tried out for the duration of Spring Ball. Typically, 1-2 made the team (mostly special teams/backups, but at least 1 starter that I can think of). It was a tough road for these guys - they were strangers in a tight-knit position group, didn't know the drills, started behind on the scheme/calls, didn't get much real consideration from the coaches, and fell last in line for playing time to show their stuff.