The Common App has little circles that you click to fill in, indicating grade(s) you participated. Fill in the circle for "12" for Beta Club. There are also boxes for you to fill in "hours per week" and "weeks per year." For a 12th grade activity, it's ok to project for these. For your junior year FBLA, you would end up putting however many hours per week you participated, as well as weeks per year, so it would be apparent if you barely participated. Be careful about listing activities you barely participated in, though, as it can look like you're trying to pad your application by listing insignificant activities.