For the most recent answer to this question and an argument for exactly how the members of the Qumran community employed the Greek language, see the latest issue of the journal Dead Sea Discoveries. Matthew Richey, “The Use of Greek at Qumran: Manuscript and Epigraphic Evidence for a Marginalized Language,” Dead Sea Discoveries 19, no. 2 (2012): 177–197.
Richey’s article is clearly written and provides a helpful summary of the available evidence. I hope the following sketch of the article's salient points will send you looking for a copy of the full article. In “The Use of Greek at Qumran,” Richey argues,
The Covenanters employed the Greek language in a clearly defined context, namely, that of economic transactions, both intramural and extramural. (178)
To support this thesis he looks at two texts (4Q350 and 3Q15, the Copper Scroll) and the epigraphic evidence recently published by André Lemaire (see below). This material is especially significant because it represents writing original to the Qumran community and the content of these writings reflects day-to-day affairs.
4Q350 (4QAccount gr)
This is the only unambiguously documentary (as opposed to literary) manuscript among the relatively small percentage of Greek fragments found at Qumran (178). Richey states,
On the whole, 4Q350 is, unlike all of the other Greek texts uncovered at Qumran, a text that participates in common Hellenistic conventions for registering quantities of goods. (183)
Some would argue that the unique nature of 4Q350 argues for a provenance outside Qumran. Richey, leaning partially on the work of Catherine M. Murphy (Wealth in the Dead Sea Scrolls and in the Qumran Community [STDJ 40; Leiden: Brill, 2002]), argues that the evidence suggests 4Q350 should be considered a genuine Qumran fragment.
Summarizing the significance of Lemaire’s work on Qumran epigraphy (“Inscriptions du Khirbet, des grottes et de ʿAïn Feshka II,” in Khirbet Qumran et Aïn Feshkha, II. Etudes d’anthropologie, de physique et de chimie [ed. Jean-Baptiste Humbert and Jan Gunneweg; Fribourg: Acodemic Press, 2003], 341-88), Richey writes,
On the whole, Lemaire’s epigraphic work has demonstrated that the use of Greek at Qumran, while not widespread in comparison to nearby first century sites, must be taken into account in any characterization of language use at the site. (188)
Richey cites Lemaire, “The use of Greek… seems rather greater in the documents of practical life than in the literary manuscripts,” (Lemaire, 381, as quoted by Richey, 188-189) and adds,
I would claim, furthermore, that our evidence suggests Greek usage at Qumran to have been connected with prospective economic dealings; some of these may have involved outsiders, many of whom would have spoken Greek as a first or second language. (189)
3Q15 (Copper Scroll)
The mysterious Greek letters at the end of certain lines of the Copper Scroll have long been noted, but there is no consensus as to what they mean. Richey summarizes the two most plausible explanations of this apparent shorthand writing. The Greek letters represent either numbers or names. He eliminates the numbers option by pointing to the work of Judah K. Lefkovits (The Copper Scroll 3Q15: A Reevaluation, a New Reading, Translation, and Commentary [STDJ 24; Leiden: Brill, 2000]), and concludes,
The inferences to draw from all of these possibilities is that some of the Covenanters spoke Greek, or they could reasonably expect one of their number to have both the opportunity and desire to learn Greek in the near future… The most intriguing correspondence between the Copper Scroll and the epigraphic material is the use of Greek for onomastic inscriptions. This manuscript usage might provide support for the conjecture that Greek was used, at least in profane contexts, to preserve the non-Semitic names of initiates. (195)
Taken together, these pieces of evidence suggest that Greek was used by the sect when they engaged in both affairs releated to trade and treasure hiding and seeking, a closely related activity. That a greater number of both Greek and Semitic documentary texts has not survived is likely the result of the conditions that prompted the Covenanters to hide their manuscripts in the first place; it is unlikely that, in the face of imminent danger, the Covenanters would deem most papyri detailing ephemeral transactions to be worthy of preservation. (197)
On top of writing very clearly about a complex topic, I think Richey makes a persuasive case. Any thoughts?