You can use an SMS gateway like http://www.clickatell.com/
It is important to notice that 2-Way SMS in the US has strict regulations - you have to use a short code, otherwise your service is considered illegal and the carriers can shut you down. ClickATel does not operate based on shortcodes, they use SMS modems which is basically a way to hook up a cellular line to a computer and intercept SMS messages.
There's quite a bit of details that one should be aware of if one wants to run a business grade SMS service in the US due to the way carriers control the SMS traffic. If there's interest from the readers I'll be more than glad to write down some of the details.
In the US, a Clickatell SMS will come from their shortcode, which is perfectly fine. No need to pay big bucks for your own shortcode, like some SMS providers want you to believe.
If you need to receive SMS, it's a different story. You could get a UK number, which is much cheaper then a US shortcode, but might look weird to your US users.