The "Instrumental Soloist" category is open without restriction to any classically trained solo instrumentalist, including guitarists, organists, percussionists and all "orchestral" instrumentalists.
There are separate categories for professionals, college/university musicians, community and high school artists. Applications on behalf of instrumental soloists may be made by a coach, teacher, manager or other person so authorized, or, in the case of students, by a parent. Only one application per soloist per year may be submitted.
The "Instrumental Performance" contest details and application form may be found with information about "The American Prize in Chamber Music" here: http://www.theamericanprize.org/ChamberPerformance.html
The postmark deadline for applications is Tuesday, March 15, 2016. There is no live competition. The American Prize evaluates recorded performances only. See the website for details.