When an article is written by several authors and each section is individually authored, how do I cite a section?

Mention the author of the section you are citing in a signal phrase. For clarity, you might indicate the name of the section in your prose, especially if an author writes more than one section:  

In “Faculty Members, Accom­modation, and Access in Higher Education,” Rosemary Garland-Thomson writes, in the section The Changing Profession: “Accommodating disability in the workplace is an opportunity to define the essence of a job.” 

The parenthetical citation must key to the article as a whole, so provide the last name of the first author of the article and then et al.: 

In “Faculty Members, Accom­modation, and Access in Higher Education,” Rosemary Garland-Thomson writes, in the section The Changing Profession: “Accommodating disability in the workplace is an opportunity to define the essence of a job” (Kerschbaum et al.).

Work Cited

Kerschbaum, Stephanie L., et al. “Faculty Members, Accom­modation, and Access in Higher Education.” Profession, Modern Language Association, 9 Dec. 2013, profession.mla.hcommons.org/2013/12/09/faculty-members-accommodation-and-access-in-higher-education/.