Call them USPCs. Log all known information on the subject into the file, such as sightings, eyewitness accounts, physical evidence.
USPCs are that, as opposed to SPCs, because they are that exactly: unconfirmed. Bigfoot, Loch Ness, anything new where the subject isn't immediately discovered, but only the idea of the subject.
Confirmation of the subject includes transferral into an established SCP file, with the description and holding procedures as is protocol, and with additional collected information attached as an addendum, marked as knowledge prior to realization of the subject.
As far as people involved are concerned, some would obviously have to be included in addendums, as SCP-xxx-01, if the person were infected or somehow affected to a point where normalcy is impossible and he is as much a danger as the SCP itself. People who have only witnessed the SCP, or those affected but still capable of normal human behavior, obviously cannot be released into the public (at least, in almost every case), but may be offered employment within the organization; either as an operative, if he is capable enough, or has abilities unique to him, perhaps afflicted upon him by an SCP, or in a more mundane sense (janitor, caf worker, etc).
As I'm running a tabletop game around the idea of the SCP Foundation, the players will be starting at the lowest levels, as new recruits, in both stats and ranking within the organization. They're first mission will be investigating the possibility of an SPC, one that seems somewhat implausible. A senior officer will be sent with him, to "babysit", as punishment for screwing up royally on a previous mission.
Demoting SCP operatives that are worthwhile and valuable to minor investigations as opposed to throwing away their skills by feeding them to a Keter class SCP seems more plausible. Think of a cop of ten years being put on traffic duty, that kind of thing.
It also makes sense for new operatives, those who aren't with the organization related specifically to certain SCP's, but fresh new recruits with a broad sense of purpose, to be eased into the life. Think of it this way; you don't take calculus in kindergarten.
Being my first post here, I hope all this falls within the ideas and boundaries that you've all set for the SCP Foundation. I'd hate to step on any toes, or look like a fool.