Complex sentence: Magnussen joined the National Socialist German Students' League (NSDStB) while she was still an undergraduate in college; by 1931 (when she was 23) she was a member of the National Socialist German Workers (Nazi) Party.
Split into simpler sentences: Magnussen joined the National Socialist German Students' League (NSDStB) while she was still an undergraduate in college. By 1931 (when she was 23), she was a member of the National Socialist German Workers (Nazi) Party.