What exactly have you looked at?
A very comprehensive run of muster rolls for the First NY Regiment (Continental) under Col Gosen VanSchaick is on Ancestry.com, as well as a smattering for its predecessor by the same name as constituted until the reorganization ("second establishment") as of late 1776. Have you looked at all of them? They are indexed (not perfectly) but no index links go to specific pages - you must browse through the images.
On both ancestry and footnote are the Rev. War Compiled Service Records, arranged alphabetically by Regiment. They will tell you dates of service and what Company an individual was in. Have you looked at them?
Since you did not give a name and vital dates, no one can help further.