1. Mingus is a USHPA regulated site. It is one of the few sites in Arizona that AZHPA (the Arizona club) decided to keep insured under the "new" USHPA system. This has nothing to do with any tandem instruction going on there, and to my knowledge there are no tandem instructors with the required PASA certification to fly tandems from Mingus. But Mingus and a few other sites in Arizona were required to have insurance by either the Forest Service or the City of Phoenix, in order to honor previous AZHPA agreements/contracts, and therefor for the time being the club was forced to keep them as named insured sites with USHPA.
Ironically, under the new USHPA system that was supposed to promote safety, USHPA instructors are not even allowed to offer any mentorship or anything that could be perceived as instruction to any current or previous students at many awesome sites like Mingus without the expensive PASA site-specific certification and RRRG insurance. In my personal opinion, this is one of the many failures of the new USHPA policies, but it is all a subject for another debate... back to the fly-in.
2. While Red's advice about o2 is sound, a few of the details about regulations are a little off. If you want to get picky about it, the FAA requires o2 for "aircraft", for the minimum flight crew (pilots), for more than 30 minutes above 12,500, or for any time above 14,000, and o2 is required for everyone on the plane when above 15,000. However, since we operate under FAR part 103 and fly "ultralight vehicles", the rest of the FAR's written for "aircraft" including the rules about o2 do not apply to us, and part 103 has no specific requirement for o2. So there is no legal requirement for o2 when flying a hang glider.
That being said, Mingus is a high mountain site with potential for high climbs. To have a good time at Mingus you need to understand all of the aspects of flying big mountain sites, including but not limited to the appropriate use of o2. The "appropriate use" of o2 will be very different for different people on different days. And in my opinion, in no way will the lack of o2 prevent you from having epic flights at Mingus. High altitude launching and landing skills are much more likely to be a determining factor of your experience flying Mingus.
My personal experience (exactly 1 flight at Mingus): Upon arriving at launch it was immediately clear to me Mingus is a diamond in the rough. It must be one of the best mountain sites in the southwest US, not just from the potential of the flying, but the fact that AZHPA maintains a fully private for-pilots-only campground, at launch, inside US forest service, at a potentially high impact area, with a private bathroom, on top of a mountain, fully set aside for ONLY pilots to camp and fly from. Let that sink in!
So in summary; go to the Mingus fly-in, all are welcome, bring o2 if you've got it, fly safe, have a good time, and SUPPORT AZHPA TO KEEP MINGUS OPEN. Mingus is awesome!