Banking
Rules and regulations for opening a bank account as a business in the US varies in each state, but you will be expected to file a SS-4 form to get a US tax ID number (EIN). A W8/W9 form is also required along with additional documentation. This is one of the more complicated aspects of expansion and is often taken care of by a US-based lawyer. It’s also worth discussing with your UK bank if they partner with an American bank.
Taxes
When you expand a business to the US, you’ll also be paying US taxes. Each state sets its own rate
of tax, so they will vary depending on where you set up as well as factors such as legal structure and earnings. This is a complicated area so always utilise a CPA or Tax Accountant to make sure that your business is compliant both in the US and your home country.
Contracts
Companies should be very careful when engaging in contracts with US clients: clauses and provisions that may appear like boilerplate around, for example, fee-shifting, venues and disputes, can contain hidden dangers and mistakes here can be costly. In the case of nonpayment, for example, while a fee you are attempting to recover may only be in the low tens of thousands, attorneys’ fees for collection could end up well in excess of that, rendering pursuit pointless. In New York, you don’t get your fees unless there’s a contractual or statutory basis for their collection.
Venue provisions are also important to note. In one case, a European client of ours agreed to the Middle District of Florida as a venue for legal recourse and ended up at a huge disadvantage against their adversary, which had employed a good percentage of the townspeople and were blessed at trial by a jury perhaps predictably unfavorable to the overseas client. Throw in the expense of local counsel and travel back and forth to Florida, and a provision that once seemed meaningless ends up being a story you wish wasn’t yours to tell.
"borrowed" from an RDLC USA Recruitment guide by Gary Goldsmith
You can get a copy here
https://landing.rdlcpirates.com/USA-ebook/nyc-form-download.html
A lot of the content applies to non recruitment companies too.