Write your paper as usual, coming to the point where you would need to cite a multi-authored source.
Signal the source. Put the information into your own words, and give the reader some background information before quoting. If you mention the lead author (by last name) and that the article was co-written, the end of the sentence citation should only contain the year of publication. For example: As Professor and his research assistants conclude, "This is a quote" (2010).
Add the surnames of the lead author and co-writers you paraphrase without contextualizing the source. For example: These are facts and figures I am paraphrasing (Professor, Anotherprofessor, & Gradstudent, 2010). After the first time you cite this source, you can revert to the Latin "et al," with the lead writer's surname--for example, (Professor et al, 2010). If there are more than five co-writers, add the Latin "et al" after the lead writer in the first citation and every subsequent citation. Example: These are facts and figures I am paraphrasing (Professor et al, 2010).
Take the lead author's name, and add the new source to your reference list. If the lead author is "R. Professor," you would go by the "P" in the author's surname.
Write out the reference list citation. It should look like this, if the journal article has seven or fewer coauthors:
Surname1, first initial, Surname2, first initial, & Surname3, first initial. (Year of publication). Article Title. Journal name. Issue number (volume number, if available). Page numbers.
Example:
Professor, R., Anotherprofessor, A., & Gradstudent, M. "The Quantitative Effects of Pez Dispensers on Mass Psychosis." The Journal of Fictional Psychology. 4(1) 23-71.
Add an ellipsis if the journal article has more than seven co-authors. However, you must list the first seven. Example:
Professor, R., Anotherprofessor, A. Gradstudent, M., Anotherperson, A. Paidresearchassistant, A. Undergrad, W., Scholar, G., . . . "The Quantitative Effects of Pez Dispensers on Mass Psychosis." The Journal of Fictional Psychology. 4(1) 23-71.