I answer this with two rule sources.
From the description of the Fly Spell...
Using a fly spell requires only as much concentration as walking, so the subject can attack or cast spells normally. The subject of a fly spell can charge but not run, and it cannot carry aloft more weight than its maximum load, plus any armor it wears. The subject gains a bonus on Fly skill checks equal to 1/2 your caster level.
From the description of the Witch's Prehensile Hair Hex...
Effect: The witch can instantly cause her hair (or even her eyebrows) to grow up to 10 feet long or to shrink to its normal length, and can manipulate her hair as if it were a limb with a Strength score equal to her Intelligence score. Her hair has reach 10 feet, and she can use it as a secondary natural attack that deals 1d3 points of damage (1d2 for a Small witch). Her hair can manipulate objects (but not weapons) as dexterously as a human hand.
The hair cannot be sundered or attacked as a separate creature. Pieces cut from the witch’s elongated hair shrink away to nothing. Using her hair does not harm the witch’s head or neck, even if she lifts something heavy with it. The witch can manipulate her hair a number of minutes each day equal to her level; these minutes do not need to be consecutive, but must be spent in 1-minute increments. A typical male witch with this hex can also manipulate his beard, mustache, or eyebrows.
With this, we can use RAW to determine that the hair functions on a completely different set of rules then the PC does. Because of the line that says that carrying something very heavy still does not strain her neck or head, we can ascertain that the hair is given it's own frame of reference to carrying things. At that point, when she's flying, the hair is also considered to be 'flying' on it's own, as if piggybacking onto her Fly spell.
But then we have to look at maximum load of the hair.
Since she has a 20 Int score, the hair's Str is 20. Maximum load for a 20 Str is 400 lbs from this source. With this, and your information that says that your party members are no more then 180 pounds (including full gear and everything) then this seems very plausible that you'd get everyone across. Though, speed may be a factor, simply because you'd be at a full load like that and would have trouble moving and flying, requiring constant Fly checks. But that's a different question.