How to make neon people in after effects?

The Tron stuff is actually mostly a matter of people running around in fancy costumes, be that the ballet leotards with painted on stripes in the old one or the more fancy LED-driven stuff in the newer one. And that's pretty much the answer. You won't be able to add those strips after the fact unless you have a gigazillion budget where you can afford to have compositors work on it for days or do it in 3D animation. So for what it's worth, this really isn't much of an AE question at all unless you make the necessary preparations like going out and buying som tight fitting clothing and painting it up with textile paint or glueing on some reflective tape. Of course the expanded process would likely still include green screen work and fancying things up with glows and such, but you need to get the basics down first and really focus on getting a good plate to start from. that's 70% of the success in this case.

Mylenium

The most important part of this kind of project is preparation. Costumes and background are the major considerations. You need a costume and makeup that will facilitate separating the footage into layers. The face and hands in the original Tron were isolated and color corrected, and the costumes were animations. Unless you have weeks or even months for the project I suggest you get some kind of a jumpsuit that you can add some white tape to so you can easily separate the costume from the hands and face on one layer, use the white tape to generate a matte for a glow effect, remove the background using keying, and if you want to keep it as simple as possible, don't move the camera at all. Then it's just a matter of separating the footage into layers and stacking up effects, mattes, and blend modes. Look at some of the original Tron images and try and figure out how many effects you see, then plan and shoot to make getting those layers as easy as possible. 

I see, starting from the top:

• A fog layer with a matte or mask covering the bottom half of the actor
• The actors head color corrected
• The actor's costume with white stripes (I know, it's digital but if it was a bodysuit with white lines on it)
• The actor's hand
• The Prop disk
• Another fog layer
• A middle ground layer with some kind of graphic
• Probably another fog layer
• The Background

I hope this gives you some ideas. 

I have no idea what "neon people" are. Sorry, but you have to be much more specific what you are actually referring to and provide a link to a reference image/ style board of your preferred look. From Tron's circuit board look to more abstract stuff this could really mean anything and the techniques on how to achieve the looks will be hugely different. A lot of it will have to begin with actually planning the shoot, creating suitable costumes and whatnot anyway, so at this point it isn't even an AE question. You have to think of the whole process, not just how to handle post production. So for all intents and purposes you need to provide much better info.

Mylenium