Both FOIL and the Open Meetings Law apply to governmental entities.  Because that is so, the New York County Democratic Committee is not subject to either of those statutes.    However, the event that you described might be required to be open to the public based on a different provision of law.   Section 1‐104 of the Election Law states that:   

"The term 'caucus' shall mean an open meeting held in a political subdivision to nominate the candidates of a political   party for public office to be elected in such subdivision at which all the enrolled voters of such party residing in such   subdivision are eligible to vote."

I am unaware of whether an assembly district can be characterized as a "political subdivision" or, therefore, whether §1‐ 104(28) is applicable, and it is suggested that the question might be raised with the State Board of Elections.  It is noted that an informal opinion of the Attorney General issued in 1977 advised that "the term 'political subdivision'....means the area from which an officer is selected and also, in the case of an elective officer, the area in which the people who elect him reside."

I hope that I have been of assistance.

Bob Freeman