Can a (druid wildshaped into a) huge air elemental carry other medium creatures while flying? How many? Can those creatures cast spells while being carried? Make ranged attacks?
Can other medium creatures ride a huge air elemental while it is flying? How many? Can those creatures cast spells while riding? Make ranged attacks?
as long as they are within the relative carrying capacity, they behave as any other creature carrying another creature would.
For spellcasting, I'd use the same concentration checks as vigorous motion while casting (probably kick that up to violent motion if in combat) and give ranged characters the same penalty as riding a horse and doing so.
Yes, that's the conclusion I came to, as well. However, the concentration check required for vigorous motion is DC 10 + spell level, which is pretty much a sure-fire success for mid- to high-level characters. And if the "mount" is only taking a single move, there's no penalty to ranged combat.
Given how little air elementals weigh the most stable arrangement of mount and riders here is with the riders below the mount. And there's no way you could make a saddle here, you're riding bareback. I might increase ride and concentration DCs by another 5 to represent that.