Well, let me take a stab at it (the question, not your cat, if you promise you aren’t stealing Jewish and Protestant babies for secret Catholic cannibalistic rituals

).
For context, I am a bona fide grownup (age 44) college educated Hellenic Neopagan Unitarian Universalist. I am also a happily married suburban homeschooling mom who doesn’t live in a fantasy world in my parents’ basement

. You are most likely to find me trying to get my lawn to grow (new subdivision), shuttling the kid to gymnastics or playdates, hitting the library or the grocery store, working on social justice projects (feeding hungry kids, promoting conservation of resources and sustainable living) or chatting with the other moms at the park. In other words, I’m pretty boring and normal. You would not look at me and automatically say “ooh, she’s a Pagan!” (as if Neopagans did not come in all ages, races, income and educational levels, walks of life, etc just like members of all other religions). Actually, many folks in my area assume I am a conservative Christian simply because I homeschool

.
I am more likely to be playing board games or checkers with my kid than D&D (tried it once–it was boring and I was awful) and would rather have good quality dark chocolate than Twinkies (revolting stuff–they and the cockroaches will be around long after any nuclear blast). I liked Harry Potter (which is no more a “how-to” or secret occult recruiting manual than Cinderella or the Wizard of Oz, despite what some Christian groups will claim), but would prefer a good mystery novel.
I will be the first to tell you that there is a lot of dreck printed under the name of Neopaganism, just like there is under the name of Christianity. It must all be looked at with a discerning (and skeptical) eye.
Are these people actually practicing some real religion(s), or do they just make this stuff up as they go along?
Well, to answer that fully, I need to have more info on what
you consider constitutes a “real religion.” I have met many more Catholics on this board than I had ever expected that are very vocal in saying that no religion other than Catholic Christianity is a real religion.
By mainstream standards, yes, the vast majority of Neopagans are practicing religions that are just as real as yours is, a Buddhist’s is, a Hindu’s is, a Jew’s is, etc. Note that I use “religions” as there is not one singular Neopagan religion. It is more of an umbrella term that in common usage encompasses a wide variety of religious traditions, some of which are as different from each other as Christianity and Buddhism, much less Catholic and Baptist. It has more in common with the term “Eastern religions” than with the idea of sects of a single religion. To get a feel for that variety, you might start with looking at
religioustolerance.org/neo_paga.htm. A couple of books that you might find useful:
“The Complete Idiot’s Guide to Paganism” (yes, I know, the title is amusing, but I have had many tell me it is a wonderful introduction) and also “Paganism: an introduction to Earth-centered religions” by Joyce and River Higginbotham. Both of these are readily available at places like Amazon, I got mine from a local chain bookstore. Neither is a “how-to” book, more of an overview of the fairly current Neopagan community in the US.
As to whether some want to “just make it up as they go along,” well, I have heard plenty of folks on this board accusing lots of other Christian groups (in some cases all) of doing just that. Sometimes they are right, sometimes they are wrong.
The same is true in the Neopagan community. Here’s an interesting post from someone following the Asatru (Norse) religion that addresses that issue from his perspective:
cauldronborn.blogspot.com/2007/03/demands-of-ancestral-religion.html
Finally, unfortunately, yes, there are some folks who will call themselves Neopagan just for shock value, but those don’t tend to stay around too long.
Are they organized or independent?
Depends on the religious tradition and sometimes on the sect within that tradition. Some have local, national or international organizations, but a great many are independent.
How many of these people are around anyhow?
Because of the above, coming up with precise numbers is difficult if not impossible.
adherents.com/rel_USA.html might give you a starting point on that one for the US.
Are they just going to sit around eating Twinkies, reading Harry Potter and playing D & D, or are they going to start killing our cats?
Not unless they were planning on doing that even if they weren’t Neopagan. This is pretty much the equivalent of saying “well, a Catholic Church is moving into our area…are they going to just sit around worshipping their statues or are they going to start molesting our children?” Just as absurd and just as offensive. The local store is likely to have a similar impact on you and your neighbors as your Catholic church has on its neighbors–not much unless you choose to go inside.