There will be at least a few runners around Boulder preparing for the Twin Cities / Chicago marathons in early October - with the majority hoping to meet the OT standard. So there are plenty of potential training partners in August/September here. However, training with these runners would not be a "camp-like" atmosphere because most of them work and have varied schedules.
That being said, I am sure someone would make room for a serious runner for 3-4 weeks (the Boulder community seems generally supportive of elite athletes). If you are worried about staying at only 5300-5500 ft, there are many possibilities within 30-40 minutes of Boulder where you can stay 8000+. Probably the most convenient without a car is Nederland because of the bus to/from Boulder. If you want to rough it, there are plenty of camping options at 9500-10,000ft close to trails.
A bit higher in elevation, Colorado Springs is definitely another good option, it is my understanding there is usually a solid group of UK runners that train there as well (although this might only be in the spring).
Although the relative cost is higher in Boulder than Flagstaff, there are obviously good reasons why the best group of runners in the world come here to train for their fall marathon (Kimbia Athletics - Evans Rutto, Paul Koech, etc.). Additionally, most of the elite Romanian and Japanese female marathoners are here in August/September, as well as Lornah Kiplaget. It is probably the combination of good weather, sunshine, and access to a wide variety of training conditions that makes it attractive to them.
So many posters on LetsRun are advocates of the high/low system. I lived in Flagstaff last year and although it is amazing for training (more trees and shelter from wind), it is actually tougher to run in Boulder for some strange reason (Barrios used to say the same thing when taking groups to high altitude in Mexico and coming back). There are plenty of training runs you can do at 8000+ feet (Magnolia, Switzerland, Sourdough, etc.), plus tons of hill training options (head to Chautauqua Park and look up West, North or South!). If the best runners in the world who live at high elevations, also train at even higher elevations then the proof is in their results. This is unlike the terrain aroung Flagstaff, where everything is relatively flat at 7000-7500 ft (unless you go up the mountain stichback, which is fun). The Fairview H.S. track facility is very good at 5500ft, although in late August I assume it will get quite busy. But there are plenty of short loops possible for regular intervals on dirt, grass or asphalt. The 800m wood-chip loop outside Potts Track is especially good for repeats. I hear many runners use Marshall Road for long intervals as well.
I would be fairly interested in helping organize a training group here in August (many friends have expressed some interest). Since moving here in March, I have found almost every runner I have met open to training with others. However, the scheduling is more difficult and most are accomplished athletes who also feel comfortable training solo. But recently, a few have expressed interest in trying to get together for training, especially with the fall marathon in mind. Send me an email (actually I think I may have a good idea who originally posted) and I will see if I can help.