Vratyas

Skt., vratya; pagans, outcasts

Little information can be found on these outcasts of early Vedic society, but what we know makes them look like proto-type Tantrics who, as was done elsewhere on the planet, worshipped the Goddess and celebrated life with wine and orgies. Women among the Vratyas sometimes became pumscali (ritual prostitutes) and they may well be the precursers of the later devadasi tradition.

Philip Rawson, referring to Vratyas but probably speaking of the pumscali, makes them sound like an all female "sect" and connects them to the dakinis and yoginis of later myth and ritual. According to Rawson, they may represent "a female line of power holders" who initiated male Tantrics by "ritual intercourse with them".
[Rawson. The Art of Tantra, p. 80]
Mircea Eliade, in his discussion of Shiva, contributes information that refers to the Vratya men. According to him, an obscure chapter of the Atharva Veda refers to this "mystical fellowship," but does not tell us much more than that they dressed in black, wore turbans, practiced yogic techniques such as breathing exercises, and "homologized their bodies with the macrocosm". Thus, he sees them as a precursor of the later ascetics and yogis of Shiva.
[Eliade. Yoga, pp. 103f., 256f.]
Indra Sinha, on the other hand, clearly defines them as non-Dravidian, Aryan outcasts who were known to have celebrated "bacchantic, orgiastic rites" and hints they may have continued traditions from the early Indus Valley civilization.
[Sinha. Great Book of Tantra, p. 72]