The rabbis can give rulings, but from talmudic times to the present it is the masses of religious Jews that determine if a halakhic ruling is accepted or not. Even with regard to the greatest poskim, you find that for some of their rulings the religious masses simply refused to accept them (perhaps because they found them too difficult and were already accustomed to do things in a different way, e.g., R. Moshe Feinstein and time clocks on Shabbat). I am not sure what Grossman’s problem is with the notion that popular acceptance can determine halakhic practice. Maybe he thinks that halakhah is only about the posek issuing a ruling. However, especially when speaking historically, we must also take into account that the religious masses (the “olam”) also have a crucial role to play in how halakhah develops.
This important notion is elaborated upon in the recent book by R. Ronen Neuwirth, The Narrow Halakhic Bridge, pp. 293ff. Here is one passage from R. Kook that R. Neuwirth quotes (from Eder he-Yekar, p. 39): “All of the mitzvot of the rabbis that we fulfill – their main foundation is the acceptance of ‘the entire nation’ which is the honor of the nation.”
…If you look at Jewish history, you will find that while many have asserted that certain beliefs are obligatory (e.g., gilgul, existence of demons, Divine Providence encompassing the animal kingdom, Daas Torah, R. Shimon Ben Yohai authored the Zohar, the Sages were infallible in matters of science), these beliefs have never become generally accepted to the extent that those who do not share them are regarded by the wider Orthodox world as outside the fold. Only Maimonides’s Principles were able to do such a thing. This explains what I mean when I say that had Maimonides not regarded the Messianic Era or Resurrection as obligatory beliefs, that “it is unlikely that denial” of them would have been enough to place the stamp of heresy on such a person, and thus to disqualify him from being a hazzan.