While the mainstream Enlightenment was definitely an elite affair conducted in public, there were other elements to it. The so-called Radical Enlightenment (the term was invented by historians, not people involved in it) had to do with the ideas too scandalous for mainstream philosophes to support, like outright atheism. One examples of this phenomenon was the emergence of Freemasonry, "secret," although not difficult to find for most male European elites, groups of like-minded Enlightenment thinkers who gathered in "lodges" to discuss philosophy, make political connections, and socialize.
Some Masonic lodges were associated with a much more widespread part of the "radical" Enlightenment: the vast underground world of illegal publishers and smugglers. In areas with relatively relaxed censorship like the Netherlands and Switzerland, numerous small printing presses operated throughout the eighteenth century, cranking out illegal literature. Some of this literature consisted of the banned works of major philosophes themselves, but much of it was simply pirated and "dumbed-down" versions of things like the Encyclopedia. This illegal industry supplied the reading public, especially the reading public with little money to spend on books, with their essential access to Enlightenment thought.
For example, as noted above, an actual volume (let alone the entire multi-volume set) of the Encyclopedia was much too expensive for a common artisan or merchant to afford. Such a person could, however, afford a pamphlet-sized, pirated copy of several of the articles from the Encyclopedia that might interest her. Likewise, many works that were clearly outside of the acceptable bounds of legal publishing at the time (including both outright attacks on Christianity as a fraud as well as a shocking amount of pornography) were published and smuggled into places like France, England, and Prussia from the underground publishing houses. Perhaps the greatest impact of the Radical Enlightenment at the time is that it made mainstream Enlightenment ideas - however poorly summarized they might have been in pirated works - more accessible to far more of European society as a whole than they would have been otherwise.