I played pickup this way in college for several years.
Like some of the others suggested, an LED disc works well (green works far better than red or blue in my experience).
We also used glow sticks, although we generally only used them on the arms because it was cheaper, an they tended to get in the way for running when on the legs.
The biggest thing I found helped though was using small battery-powered road flares for marking end zones.
Ones with LEDs can be seen all the way down a regulation field.
Biggest caution is watch out for holes in the day before you play, as injuries from twisted ankles were far more common at night.
outdoors.stackexchange.com/users/4003/SSumner
Why all the lights?
you could solve everything you need and have a REALLY cool event with nothing but a lot of glow sticks :)
Make people wear different colors for different teams, then one each side of the body for arms, hips and legs (so 6 glow sticks per person).
That should be more than enough for a good time out there.
outdoors.stackexchange.com/users/3624/Unknown Coder
With ultimate specifically, traditional outdoor lighting methods won't work well.
Flashlights are right-out, because you need both hands to play.
Headlamps are a bad idea because you'll blind your teammates.
Lanterns don't cast light far enough to illuminate a whole field.
Glowsticks work well for illuminating players, as pointed out above.
For illuminating field markings, such as goals and boundaries, you can use a variety of stationary lights.
A flashlight inside
a gallon milk jug creates a nice diffuse light that's easily visible but not blinding.
You could use those for markers, as it would be cheaper (and safer probably) than buying full-fledged lanterns.
outdoors.stackexchange.com/users/208/nhinkle
Having actually tried this on a campsite I would advise you to play in a well lit area.
It turns out throwing spinning plastic discs at people in low light can actually be quite dangerous!
I would suggest: A well lit area Flat ground (checked during the day, you don't want anything to trip over).
Ideally some kind of glowstick/band to indicate teams clearly If you can get a frisbee which has it's own lights that would work best
(I'm not sure how good a frisbee it would be though).
As mentioned in other answers lighting should come from external forces not headlights, because you look at other players to throw the frisbee to them, this results in the intended recipient being dazzled while they're trying to catch!
outdoors.stackexchange.com/users/3322/Liath
Glow sticks and an LED disc are great ideas.
Another option could be glowing body paint ; looks like fun :)
That game is also on a beach which means less concerns about twisted ankles
outdoors.stackexchange.com/users/4006/craq
Suppose I want to play a 10-people outdoor ultimate frisbee game at night, with no power outlets available at the field. I expect it to last no more than 2 hours.   What kind of portable power supply should I buy?  What kind of lighting should I be looking for?  How do I know the light and power supply are compatible and how long it will last?
What lighting equipment do I need for playing ultimate frisbee at night?
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<gear><night>
outdoors.stackexchange.com/questions/6825
outdoors.stackexchange.com/users/3998/Russell
A variety of stationary lights could be used. Small battery-powered road flares are recommended for marking end zones, as traditional lighting methods are reported to not work well. Glow sticks and an LED disc are popular suggestions. While it is best to play in a well-lit area, stationary lights, glowsticks, LED discs and bodypaint can help.
Stationary lights help, though it is preferable to play in a well-lit area. Glowsticks, bodypaint, LED discs can help with lighting.