This is not an app that you will find in the App Store. A previous app called iTether was removed from the App Store after only 20 hours because Apple did not like the app's use of the tethering feature that cellular providers charge around $20 a month for. Because the app was removed, the developers had to think outside the box in order to offer this feature to iPhone users. To do this, they created an HTML 5 version of the app. For the details, go to tether.com/iphone.
The service costs $30 a year as opposed to $20 a month, saving you a possible $210 a year. There is also a promotional/introductory price of $15 for the first year.
I have not signed up for this service because I do not have an iPhone, but if anyone tries it out they could let others know how accessible the web portal is for getting the tether to work.