I work in this field, so let me give you some really general information.
No matter what you're selling (shares, interests, etc.), it's still a security, and to comply with the law, you will need to register the security with each state you plan to offer it in. The regulatory hurdle for registering securities is high, at least much higher than I anticipate you want to deal with.
There are ways to legally sell securities without registering. The most common way is through an exemption under Regulation D. There are three exemptions under Reg D: Rule 504, 505, and 506. Each carries different requirements that must be met, otherwise you blow your exemption and are selling an unregistered security, which the SEC and the states take seriously. They regularly and effectively punish people doing so for the safety of the public.
As an example, under Rule 504 of Regulation D, you can raise up to one million dollars, but all your investors must be "accredited" investors, which means they have a net worth of at least one million dollars excluding their primary residence (so $1M not counting their home), or they must have an annual income of more than $200,000. So we're talking about rich people. Also, I believe there is a ban on general solicitation for investors, which means you can't advertise to the public. And this is just the federal rule, in addition, you will have to comply with any state rules that are in place for each state you offer your security in.
There are different requirements under Rule 505 and 506, but 504 is probably the least stringent. To say the least, to fulfill the legal obligations of selling a security, you would have to jump through a lot of hoops, and the expense to get everything in order would be high, as you would need to hire a securities law firm to help you navigate the regulatory landscape.
I am not a lawyer and nothing in this post should be construed as legal advice.
What about some guys getting together and building a business? Whats the difference there then? Or lets say 10 guys put their money together and buy one of those bitcoin ATM's. They would all own it but it doesnt sound like its something like a security then. Is the difference that the shares of the company cant be sold so easily? Or that there is no issuer? But even when you are a partly owner of a business you would be able to sell your part of the business. And i cant believe that you have to apply for SEC rules only for creating a business with others. Though im no american so...
It gets pretty granular and specific to the situation, but you're talking about an exemption from the rules in this case. Very generally speaking, there is a litmus test the Supreme Court established a long time ago to help determine whether or not the investment is a "security."
If the venture has the following four characteristics, it is generally a "security":
1. investment of money due to
2. an expectation of profits arising from
3. a common enterprise
4. which depends solely on the efforts of a promoter or third party
Most likely, the situation you're talking about would not meet the 4th criteria because people pooling money together to start a business would be active participants in the business, and not be third parties or outsiders who expect to profit off the work of the people running the business. That's generally the difference between owning a part of a business and owning a security.
The rules differ for every state, and by the specifics of the situation.