I think you're already making a mistake in thinking about it as 'entities' and 'paranormal'. Liber O vel Manus et Sagittae sub figura VI has a great bit about this:
> 2. In this book it is spoken of the Sephiroth and the Paths; of Spirits and Conjurations; of Gods, Spheres, Planes, and many other things which may or may not exist. It is immaterial whether these exist or not. By doing certain things certain results will follow; students are most earnestly warned against attributing objective reality or philosophic validity to any of them.
Which combined with the Mathers/Crowley Goetia gives more insight into a different perspective:
> The spirits of the Goetia are portions of the human brain.
Would you question the intentions of parts of your own brain? :) Maybe! But this gives a pretty different perspective on them than external 'entities' influencing you. In the end, all that matters is that note from Liber O -- by doing certain things, certain results will occur. Whether it is external entities or parts of your own brain is kinda irrelevant to causing results.
What's membership actually look like?
I'd consider joining in a secular/political sense, but I have no interest in hanging out with people who believe in and actually praise a "Satan" deity.
You will hear people say "Hail Satan!" a lot, but it's meant more in the sense of what Satan stands for from the Satanic viewpoint, which is, among many other things, primarily freedom and knowledge.
I do believe in their tenets myself, I think they're remarkably well distilled principles. I think the first and fourth tenets are especially important for the current times.
I think what you'll find a lot of Satanists believe is "an it harm none, do what ye will", and you'll find that they take that principle more seriously than others who espouse that, such as Wicca.
Anton loved Ayn Rand. Many TST members are appalled at Rand's ethical ideology, or lack thereof, myself especially included.
You'll find that in many areas, The Satanic Temple is diametrically opposed, and represents a much more coherent, logical, refined ideology, and wholly rejects social darwinism as mostly discredited, advocating instead for empathy and compassion, recognizing their evolutionary benefit.
What both organizations have in common is the demand for rationalist thinking, the belief in the immorality of God, the belief in personal freedom, and deriving their ideologies partly from the sciences of their respective time periods.