We aren't entirely sure who the Qumran group was, though one of the leading theories is that these are the Essene sect mentioned by Josephus in his writings. Josephus mentions several Jewish sects that existed in the first century, the Pharisees, the Sadducee, the Zealots, and the Essenes. The Jewish philosopher Philo of Alexandria also mentions yet another group, the Therapeutae.
The group at Qumran may have been Essene, but we aren't entirely sure.
What we do know is that the Qumran sect rejected mainstream Judaism (both of the Pharisees and Sadducees) believing that it had become corrupted at sometime by a figure known as the "Wicked Priest", some hypothesize that this "Wicked Priest" may have been Jonathan Maccabeus, who helped establish the rebellion against the Seleucid Empire and founded the Hasmonean Dynasty. Jonathan also took on the role as the High Priest of Israel. This was a highly controversial thing to do, at no point in Jewish history had the same person fill the role as both the king and the high priest. So the hypothesis goes, some Jews retreated away into the desert, rejecting what they saw as a corruption of Judaism under the Hasmoneans. Their leader being "The Teacher of Righteousness" found in DSS.
The Dead Sea Scrolls were copied or written by the Qumran community, as such it includes a number of books, both biblical and not; and also includes unique writings by the Qumran community. The Book of Giants is an example of one of those works produced by the Qumran community, and is related to the Enoch tradition (they also preserved the Book of Enoch, a 2nd Temple Period work written probably sometime around 300-200 BC). Since it is a product of the Qumran community there's simply no reason to take it as authoritative or as anything other than as an interesting work by a rather obscure 1st century Jewish sect.
-CryptoLutheran
