Q. How do I cite a poster with multiple authors in AMA (American Medical Association) style?

To cite a poster with multiple authors, cite it like unpublished material. 

Greenspan A, Eerdekens M, Mahmoud R. Is there an increased rate of cerebrovascular events among dementia patients? Poster presented at: 24th Congress of the Collegium Internationale Neuro-Psychopharmacologicum (CINP); June 20–24, 2004; Paris, France.

This applies for 2-6 authors. For seven or more authors, include the first three authors and replace the rest with et al.

*Citation example is from the 11th edition of the American Manual of Style.