Essenes was a Hebrew sect like the Pharisees and the Sadducees in first century Israel. But they are not mentioned in New Testament. Josephus mentions about Essenes in his Judean Wars and Antiquities of Judeans in surviving Greek versions.
The establishment of the sect of Pharisees, the Sadducees, and the Essenes was when Jonathan (the brother of Judas Maccabeus) was in power.
"At this time there were three sects among the Judeans, who had different opinions concerning human actions; the one was called the sect of the Pharisees, another the sect of the Sadducees, and the other the sect of the Essens." - Antiquities of Judeans XIII.V.IX.
Josephus points out that Essenes had their own doctrine instead of Old Testament and they did not offer sacrifices which can be read below.
Josephus on Essenes and on the customs and doctrine of Essenes
Josephus' Judean Wars Book II.VIII.XIII - "Moreover, there is another order of Essens, who agree with the rest as to their way of living, and customs, and laws, but differ from them in the point of marriage, as thinking that by not marrying they cut off the principal part of human life, which is the prospect of succession; nay, rather, that if all men should be of the same opinion, the whole race of mankind would fail. However, they try their spouses for three years; and if they find that they have their natural purgations thrice, as trials that they are likely to be fruitful, they then actually marry them. But they do not use to accompany with their wives when they are with child, as a demonstration that they do not many out of regard to pleasure, but for the sake of posterity. Now the women go into the baths with some of their garments on, as the men do with somewhat girded about them. And these are the customs of this order of Essens."
Josephus' Antiquities of Judeans Book 18. Chapter 1. Paragraph 5 - "The doctrine of the Essens is this: That all things are best ascribed to God. They teach the immortality of souls, and esteem that the rewards of righteousness are to be earnestly striven for; and when they send what they have dedicated to God into the temple, they do not offer sacrifices (3) because they have more pure lustrations of their own; on which account they are excluded from the common court of the temple, but offer their sacrifices themselves; yet is their course of life better than that of other men; and they entirely addict themselves to husbandry. It also deserves our admiration, how much they exceed all other men that addict themselves to virtue, and this in righteousness; and indeed to such a degree, that as it hath never appeared among any other men, neither Greeks nor barbarians, no, not for a little time, so hath it endured a long while among them. This is demonstrated by that institution of theirs, which will not suffer any thing to hinder them from having all things in common; so that a rich man enjoys no more of his own wealth than he who hath nothing at all. There are about four thousand men that live in this way, and neither marry wives, nor are desirous to keep servants; as thinking the latter tempts men to be unjust, and the former gives the handle to domestic quarrels; but as they live by themselves, they minister one to another. They also appoint certain stewards to receive the incomes of their revenues, and of the fruits of the ground; such as are good men and priests, who are to get their corn and their food ready for them. They none of them differ from others of the Essens in their way of living, but do the most resemble those Dacae who are called Polistae [dwellers in cities]."