Since, the verse BG 16.8 is in continuation of BG 16.6-7, the term "they" means "those with demonic nature". So your Qn boils down to:
Do people of demonic nature belong to particular sect?
Ofcourse No. It's not related to groups of people being bad/good or sinful/sinless. There are verses in Gita, which says that even the worst among the humans, could be considered "SAdhu", if he/she has glued the devotion within the supreme Self.
As a rule of thumb from the natural observation of Gita, those with increased qualities of Tamas (ignorance), catalysed with Rajas (passion), have demoniacal nature. While Sattva is towards divine nature.
The qualities of Sattva, Rajas, Tamas are explained in chapter 14 & 18 with good detail. For example, demonic people are possessed with the "singular & binding" qualities like:
- Pride, arrogance, showoff, rudeness, (Rajas) & ignorance (Tamas); [BG 16.4]
- They don't know what to do (Dharma) and what not to do (Adharma)
[BG 16.70, 18.31];
- Lack purity in mental & physical aspects (lack of Sattva)
- Hurtful to others [BG 16.13-15, 17.6]
In nutshell, Those with divine nature, have the tendency of liberation, the people with demonic nature tend to remain bound within certain plane of world, to consume various colours of life/afterlife.
BG 16.5 - The divine quality is [for] the Liberation; The demonic [quality] is considered as bondage ...
Note that, any kind of loka, where existence is possible -- is a bondage. Because every'thing' has all the 3 modes (Sattva, Rajas, Tamas) present within them [BG 18.40].
Though Gita doesn't explicitly say, but it's quite possible that the certain groups of people may have increased Rajas/Tamas. And it won't be surprising, if the atheists, chArvakas & even vaisheshikas may have more demonic nature and less divine nature.
BG 16.8 is slightly mistranslated by Gambhirananda (because, Asatyam ≠ Unreal). I would recommend to refer Sivananda's (Guru of Gambhirananda) translation in this case.
Here, how it can be translated word to word:
BG 16.8 - Without [any] truth, without [any] foundation, "they" call the universe with no Supreme; Without any mutuality, [it's] born from the purpose of what else but lust.
The above verse shows the mix of Tamas & Rajas among those with demonic nature:
- Tamas: "They", don't have any truth or foundation. Yet they call the world without any "God" (the supreme one). Because in the immediate observation they don't see the God, they come to conclusion that there is no God. Shortsightedness is a predominant trait of ignorance viz. Tamas
- Rajas: "They" also believe that, world is not born or progressed with mutuality founded by Dharma; It's pure game of lust. Like dumb animals, 2 people got attracted to each other and thus they bore a child. On such basis of mere probability, the world is born. Lust is born of Rajas.
Everyone has Tamas/Rajas within them, as part of Prakruti. The predominance of these modes, would lead to demonic nature.