In the US, you're already a sole proprietorship without any registration. Of course, if you're actually doing business, you probably want the protections of an LLC, which requires registration and fees. But you can accept uncompensated gifts as an individual, and you can accept business income as a sole proprietor, all without registering for anything in particular. You are, as always, required to file accurate taxes, including estimated withholding.
Number of employees or members isn’t relevant. Types of behavior performed is. It is worth talking to a lawyer about this if you have enough revenue this matters.
You can take donations but AFAIK business licenses are required in every state in the US whether you provide product, service or something online. If you take donations then by law you have to declare that as income.
There is no generally “business license” in Arkansas. Some municipalities require them (but have very limited means of enforcing that requirement), and some specific occupations require licensure, but unless you fall under one of those narrow categories you only have to file taxes.
I've always found it deliciously ironic that the Articles of Confederation and the Constitution both dealt with interstate commerce and set up the federal government as superseding the states in this regard... but then corporations are a state level entity not federal, complete with 50 states worth of regulations.
