Pro/am porn isn't all that popular these days, but there used to be porn series that were built around picking up amateurs to perform sex acts with professional pornstars -- the Shane's World College Invasion series is a good example. They would recruit college girls (off camera) to fuck one of their studs -- typically those girls would never be seen in porn again (I think they might have even blurred their faces, but I forget). Another variant was to have professional girls go out and recruit amateur guys for immediate sex acts -- they would literally walk up to a group of guys on the beach and ask if they'd like to receive a handjob, and then cut to the girl and one of the guys, with them still in their swimsuits walking into a motel room, where she jerks him off.
My impression is there are limits, though, to how this can be done, primarily due to STD testing regulations on the porn industry. In the case of the amateur girl above, she no doubt had to get tested before doing the scene, so there was a delay between when they first recruited her and when she did the scene. But in the example of the guy above, I get the impression that they can get away with more immediate filming, so long as there's no penetration.
Anyways, I'm wondering if anyone knows for sure what limits productions in going out with a hand-held camera to a dance club or beach and offering amateurs money to immediately film them doing something.