PTO's are not any part of government like cities or school boards, so I wouldn't expect there to be any 'requirement' that committee meetings be announced and open.
Since you are likely to always want any willing volunteers, making the meeting information available makes sense. But that doesn't mean the committee can't meet if it wasn't publicly announced in advance.
We've always held regular board meetings and publicized them extensively, but now we are having to have committee meetings and to increase involvement I think we should publicize those too?
Is there an open meetings act that addresses this?