I could see two ways that you go about creating NPCs based on real-life people as you describe:
- Make the NPC so that his or her abilities, appearance, and general combat effectiveness match those of the person in real life
- Make the NPC an idealized version of the person in real life, such that they are recognizable, but may have unrealistic abilities, appearance, and general combat effectiveness.
If you're going the first route, you're severely limited by several factors. First, we're all Human, so you're not going to use any diverse races. Second, if your gaming group is like mine, it's unlikely that they are very good at combat, so they aren't going to be effective in a fight. Third, even if they are decent athletes or have some experience fighting (perhaps they LARP with foam swords), you're going to be limited by the fact that everyone has the Martial power source, meaning you'll be limited to Rogues, Fighters, Rangers, and Warlords.
Going this route can work, but you're probably better off finding a different system to model these people (some like Outbreak Undead are specifically designed to be able to model real life people).
I'm guessing the second route is more what you are wanting where NPCs are idealized versions of real life people. In that case, pick a few defining characteristics of the person and choose a Race and Class based on him. Maybe Mark is a short guy with a beard who is a crack shot at the rifle range, so you make him a Dwarf Ranger. Or Kate is a hippie at heart and has long golden hair, so she's an Eladrin Druid.
I assume that since you want them as an NPC, you're not wanting to go through character creation with these folks and would prefer a statblock like the monsters. Unfortunately, the Monster Manuals for Dungeons & Dragons 4e are rather sparse on humanoids, so unless you find something in there that fits the character, you'll probably have to make some adjustments.
I'd suggest looking at a few similar entries and synthesizing some of them. I don't have my books handy right now, but you might combine elements from the entries for a generic "Eladrin" and throw in some defining elements from a Druid and you're set. Especially for races that were once monsters, you could possibly apply a monster template to a general statblock for a class.
Another option is modifying an NPC to fit the character. If you've got a DDI subscription, the Monster Builder would be great at this as it has just about every NPC ever made.
Or you could just make up stats for what looks right. That's personally what I'd do because I'm a freewheeling GM like that and I don't really care about how much XP Sam is worth, but the above methods would work better for those who prefer to go by the book.
The most important part of creating an NPC based on a person for any system is making them act like the person they are representing. Maybe they talk a certain way or they have similar motives or behaviors. Caricatures of these aspects can make them more easily identifiable, so long as the person sees it in good light.