There are several commandments that contain the phrase "b'amecha" (or a variation thereof). The gemarah darshans "b'oseh ma'aseh amcha" that those commandments only apply to a Jew who is acting in a way that is in keeping with the Jewish nation. Would Conservative, Reform and formerly-Orthodox ("off the derech") Jews be considered "oseh ma'aseh amcha" and if not are they therefore precluded from all of those commandments eg. one may speak lashon hara about them etc?
Strictly speaking such a question would have to be addressed on a case by case basis, as the criteria are not straightforward and different areas of halacha don't always use the same criteria.
Assessing a Jew's halachic status within the Jewish community depends on a wide range of factors of which the objective fact of his observance (or lack thereof) is only the first step. Much depends on exactly what he fails to observe, how he fails to observe, and why he fails to observe. Moreover, the "why" question includes both the non-observant Jew's stated reasons and our (i.e. the religious community's) understanding of why he fails to observe, even if they are in conflict. Thus, the classic case of a "tinok sh'nishba" ("captured child"), referring to a Jew who was raised in a non-Jewish environment. Such a Jew may believe that his non-observance derives from a rejection of Judaism, but we may well conclude that his non-observance is simply the result of his environment.
In general, the general thrust of most modern poskim (going back at least to the Chazon Ish) is to be very hesitant to exclude a non-observant Jew from his full status within the community.
Rav Ettlinger in the Binyan Tzion suggests two ways of being inclusive of those who are not observant. The first is to say that the children of the reformers are tinbokot she'nishbu (children who were taken captive and didnt know any better). The second is to put them into the category of Omrei Mutar (those who claim a certain action is permitted despite it being forbidden)
In both of these categories those who are not observant can be considered Shogegim or even anussim in certain cases (mistaken transgressors or transgressors who had no ability to avoid transgression)
Despite this a simple read of Rambam Hilkhot Teshuva would indicate that these people are not. What Rav Ettlinger does is explain why the modern Jew who is not frum does not fall into the categories which the Rambam describes.
See this article which is very well written on the subject