OED 'coven', noun

"An assembly, meeting, or company. Obs.

?a1513   W. Dunbar Poems (1998) 196   Lat anis the cop ga round about, And wyn the covanis banesoun.

spec. A gathering of witches; a ‘convent’ or company of thirteen witches; cf. convent n. 1, 2.

1662   in R. Pitcairn Criminal Trials Scotl. III. 606   Ther vold meit bot sometymes a Coven..Ther is threttein persones in ilk Coeven.

1830   Scott Lett. Demonol. & Witchcraft ix. 286   The witches of Auldearne..were told off into squads, or Covines.

1886   C. Rogers Social Life Scotl. III. xx. 278   To their covens or gatherings the foul sisterhood were borne through the air."

'Coven', meaning a gathering of witches, is a Scottish word, then, entering English at a late date. This is an original and not as yet updated OED entry, from 1893.

The OED is not at its best on this word form, for the very well attested sense 'coven' = fraud is not covered in its range of forms for 'cozen': "Forms:  15–18 cozen; also 15 cooson, coosin, ( cousinge, cossen, cussen), 15–16 coosen, cosen, coson,cousin, 16 cosin, cozon, coozen, cousen, couson, couzin, 16–17 couzen."

'Coven' with this meaning appears in early word lists: in Robert Cawdry's A table alphabeticall contayning and teaching the true writing and vnderstanding of hard vsuall English wordes gives "couen, fraud deceit" (1609).

An example or two of this usage: Thomas Phillips, writing about false Messiahs, in 1639: "what should the Church doe in such cases? how should shee discover the Coven and prestigious impostures of such, but by the Written Word?"

Sir Richard Hutton's The first part of the young clerks guide, or, An exact collection of choice English presidents according to the best forms now used for all sorts of indentures, letters of atturney, releases, conditions &c. (1649) illustrates multiple examples of a correct formula for trainee clerks to use :"without fraud or coven".

The other homophone of 'coven' was of course a common variant on 'convent', as in the form "Coven garden":

"Then have we Bedford Berry, commonly called the Coven garden, because there was a large Convent, or Monastery there in times pass'd, where there are many good structures, cloystered underneath some of them, with a large Piazza or Market place, and a Church that bears the name of Saint Paul, which, though within the Precincts of Saint Martins Parish, yet by Act of Parliament, it is now exempted", in James Howell, Londinopolis an historicall discourse or perlustration of the city of London (1657).

So, the senses 'coven=fraud' and 'coven=convent' are in wide usage. There are early references to a "convent" of witches. The OED cites John Gaule, in 1652 (this is the whole anecdote,for the sake of its witchcraft content):

"A certain Praetor or Judge, having sentenced divers malefactors to death, at the accusation of an Ariolist or Pythian vaticinator: at length he took upon him to tell him of one more, if he would not take it ill: the Judge earnest to know who it was, he insimulated his own wife, and prefixt an houre wherein he would shew him herin the convent of other Witches. But he (knowing his own wives integrity, and mistrusting the others calumny) at the time appointed had invited (unknown to the Ariolist) a many of his kindred and friends to suppe with his wife and him. And as they sate at supper, he took an occasion to rise, and goe with the Ariolist to the place, where he shewed him (in a spectrous apparition) his own wife in the company of other Lamian hagges. Enough to have deluded him, had he not returned, and found his wife at the table where he left her, with the testimony of all those at the table, that she had never stirred thence. Whereupon he caused the Ariolist himselfe to be executed."

So, when does 'coven' meaning 'a gathering of witches' appear first in English?

My candidate is in George Garden's An apology for M. Antonia Bourignon in four parts, published in London in 1699:

"About the end of that time, an old Woman of Lisle importun'd A[ntonia] B[ourignon] to take into the House a Girl of Nine Years, who being discovered to be one of the Coven, was immediately thrust out again, telling the old Woman that all their Secrets were discovered to the Regent of the Hospital. She run about immediately to the Magistrates, and the Parents of the Children, telling how their Reputation was quite broken by A. B. by saying they were Witches."

George Garden was Scottish. He is in the ODNB: see Stuart Handley, ‘Garden, George (1649–1733)’, Oxford Dictionary of National Biography, Oxford University Press, 2004; online edn, Oct 2007.  

The source is Garden's hagiographic life of the visionary and ascetic Antoinette Bourignon (1616-80)

https://en.wikipedia.org/wiki/Antoinette_Bourignon

Though she features in Phyllis Mack's Visionary Women: Ecstatic Prophecy in Seventeenth-Century England in a more positive light, Antoinette Bourignon's record is blotted by her behaviour as regent in a girl's orphanage she had established in Lille in 1653. This lasted to 1662. Bourignon, in this case acting pretty much like a dangerous religious maniac left in charge of young people, had discovered that all the girls under her charge had made pacts with the devil. Her regime was harsh, and the girls enduring it were not driven by the same level of asceticism and morbid fear of sex as their regent. One had died after being whipped. The rest probably spoke as they had been taught about the temptations of the devil, but found themselves taken very literally. The case is an interesting one, and I will return to it in a later post. Despite the late date, George Garden has absolutely no problems with Bourignon's discovery that her orphans were all members of a coven. A margin note on the relevant pages (292-3) reads "No grounds to disbelieve this Story, or that the World swarms with such".

After the old woman alerted other the magistrates, and they started to investigate with a bit more common sense than the various pastors Bourignon had called in, Bourignon decamped, and took off to Ghent. Among her later successes was persuading the pioneer microscopist Jan Swammerdam to renounce scientific study.