Is 'Krias Hatorah' an obligation on the individual or is it an obligation on the Minyan?
The short answer:
According to Hagaon Rav Chaim of Brisk, zt'l as quoted by his Talmid, the Birkas Shmuel, and his grandson Maran Harav Y.D. Halevi Soloveitchik, Zt'l (Boston) it's an obligation on the individual.
According to The Gr'a as explained by the Peulas Sachir it is an obligation on the Minyan.
The Chofetz Chaim in his Mishna Berura...135 Orach Chaim writes that it an obligation on the Minyan and NOT on the individual.
In Conclusion: Maran Harav SZ Auerbach,zt'l writes that it is an obligation on the Minyan NOT on the individual.
The Great Gaon Maran Harav Yosef Eliyahu Henkin, zt'l writes in his sefer "Lev Ibra" that if one goes to the washroom during Krias Hatorah and as a result misses part of Krias Hatorah he does NOT need to make up the part missed as Krias Hatorah is a 'Chovas Tzibur'.
This of course does not include Parshas Zachor which according to all is a personal obligation.