A friend of mine recently applied for Global Entry based on my recommendation, but he decided to use a company that offers to "process" the application for a fee of $130, which is on top of the $100 that the CBP charges for GE. I think the company popped up first on his Google search, so he apparently assumed that this was the official portal for the Global Entry application process. This service is not required and cannot expedite your application through the CBP system nor guarantee you an immmediate appointment-you can apply yourself on the CBP web site and pay the fee directly to them after you fill out the online form, then once you receive your conditional approval, you can search the appointment calendar and make your own appointment.
To repeat-you DO NOT need to pay for anyone to "process" your GE application - there is no expedited service similar to that for passports offered by the CBP (at least at this time), so all these companies can do for you is fill out the form for you and set up the face-to-face interview with the CBP (which is the last step before final approval). There are numerous blogs about the application itself, as well as the overall process, so if one reads these beforehand, you will be adequately prepared to do this yourself.