Requirements vary from bureau to bureau, but generally there are minimums required in order to establish a contract that governs how and what you report, as a creditor. These contracts are sometimes called Reporting Agreements or Data Furnisher Agreements. For instance, as a creditor interested in supplying data to a bureau, you may be required to maintain at least 500 accounts before you're able to obtain a direct connection to the bureau. An individual making a single p2p loan will almost certainly not meet these requirements, and will almost certainly not be able to support the software infrastructure required to do the reporting.
The good news is, most bureaus maintain separate arrangements with consolidation services, where the service acts as a third party to report on behalf of creditors who are too small to report directly on their own. These services typically charge their clients on a per month basis, and for a small number of accounts the charge will likely be a few hundred dollars a month. Bureaus still impose requirements on creditors, even when reporting through a third party service - and, again, the requirements vary from bureau to bureau.
Cases where these consolidation services are used are similar to the situation you're describing. For instance, if a real estate developer has 20 parcels of land for sale in a development, and they want to hold the mortgages for the buyers of those parcels, they would likely use a reporting service versus reporting directly themselves. However, you have to weigh the benefit of reporting the credit against the cost, and you have to make sure your lending meets the actual requirements as well - at the least, you'll need to prove that the debt is legitimate, which helps prevent abuse of the system (with no checks in place, people could easily "buy" better credit scores by reporting debts that didn't exist as being in good standing).
In order to avoid turning this into an off topic shopping-list question, I won't link to or list services here, but you can find them pretty easily with google.