New members can meet the requirement based on when they joined. For instance:

 Example #1. If they joined the previous year in January through March, they would qualify by having flown six missions in the previous year.

Example #2. If they joined April through June, they would qualify by having flown four  missions in the previous year.

Example #3. If they joined July through September, they would qualify by having flown two missions in the previous year.

Example #4. If they joined October through December, they would qualify by having flown one mission in the previous year.