Yabia Omer, there is an argument between the Rabbenu Taam and the Rambam if the saying of an unnecessary brocho is assur biblically because of Lo Siso which is the view of the Rambam see SA O’CH 215,4 or rabbinically as the Rabbenu Taam see the Tosafos Rosh Hashana 33,1. If women make a brocho on a time dependent mitzva is related to this argument. Halel on Rosh Chodesh is only a custom. so according to the Mechaber above a brocho should not be made. In SA O’CH 422,2 according to one view holds that in tzibur you do make a brocho. Could be that this view holds that Halel betzibur is not only a minhag but parsimei nisei.
