Take a look at the entry:

SADDLEBAGS Price 4 gp; Weight 8 lbs. These sturdy, weatherproof bags are draped over a saddle to add extra carrying capacity. Each side of a saddlebag can typically carry 20 pound of items that can fit in the bags. Saddlebags do not increase the amount of weight a mount can carry; they merely give riders a place to stow their equipment. Source (PRD)

The RAW for saddlebags states that they "...do not increase the amount of weight a mount can carry." The saddlebags themselves weight 8 lbs. and can hold up to 20 lbs. worth of loot on either side. This means that, at max., a saddlebag can weigh as much as 48 lbs. Also take notice of the bolded text (my emphasis). You drape saddlebags over a saddle, and a horse can only wear one saddle. This means one set of saddlebags.

In terms of other options, you also have the pack saddle:

Pack Saddle: A pack saddle holds gear and supplies, but not a rider. It holds as much gear as the mount can carry. Source (PRD)

Now down to the nitty gritty of the answer:

A light horse has a strength of 16, and a heavy horse has a strength of 20. This means that they have a maximum carry load of 690 lbs and 1200 lbs., respectively*. But be careful of encumberance; this includes your weight if you're riding it!

You also have the various land vehicles, including wagons.

As a further option perhaps you could convince your GM to allow you to use a Handy Haversack as a sort of Saddlebag of Holding? Or simply create one yourself using Craft Wondrous Items.

*: Horses are large quadrupeds and receive a multiplier on their carry capacity accordingly.

For your players' specific choices:

P1 has the right idea by picking a SAD class in the Druid, but has his dump stats off kilter. While dumping Strength, even down to 6, can be managed (especially for a Druid who should be able to travel light, as Druid defensive buffs are excellent and they are well-equipped to not need tons of rations, etal), he shouldn't put his racial +2 into it.

Given his array, I would have him dump Strength and Charisma -- Dexterity is needed for initiative, and Intelligence for skill points, so you can't dump either it turns out. Wild Shape is also more beneficial for P1 than he thinks -- Natural Spell comes at the same level as Wild Shape itself in PF, so he gets casting-while-shaped essentially "for free". Along with that, he should put that +2 racial bonus into Wisdom and bask in the glow of a +4 starting Wisdom bonus.

P5 is also on the right track with focusing on archery -- the casting of the Ranger is not strong, and with two full Vancian casters in your party (namely, the druid and the wizard), his casting won't be needed for much, while the Dexterity-based skills will be quite useful in a rogue-less party. Furthermore, his choice of Charisma for a dump stat is a reasonable one -- it's not nearly as severe a dump as in P1's case, and many Rangers don't rely upon Handle Animal the way Druids do.

Regarding concerns about healing

Having played a Druid in a party where that was the sole access the party had to healing magic, by the way, I would strongly recommend your party obtain Wands of lesser vigor if you are willing to waive the [evil] descriptor on the spell or Boots of Earth (if Fast Healing 1 is enough) if they aren't, and give one to the druid and one to another character -- Druids can't convert to healing spells as they get Summon Nature's Ally instead, and spell slots are precious especially at lower levels.

In general:

You are right to be somewhat concerned about this...

The more relevant sum in this case is of attribute modifiers -- if it's less than 0, D&D 3.5e (and presumably PF (Pathfinder) as well as it's derived from D&D 3.5e -- if it's not a rule there, you can always port the 3.5e rule to your game as a houserule) has a rule that allows the player to reroll for their stat array.

In your case, it comes out as follows:

• P1: +3, +2, +2, +0, -2, -3 = +2
• P2: +3, +2, +2, +2, +1, -1 = +9
• P3: +3, +2, +2, +1, +1, +0 = +9
• P4: +3, +2, +2, +1, +0, -1 = +7
• P5: +3, +0, +0, +0, +0, -1 = +2

Which means that all of these arrays are acceptable, at least by the sum-of-modifiers rule; however, the +2 arrays are indeed non-trivial to work with.

However, character class and construction has much more to do with this than ability scores alone.

Character classes and builds in D&D generally depend on one or more attributes to do their job:

• For a Fighter, this is either Strength or Dexterity, but Constitution also helps
• Rogues are all about Dexterity, and can make use of Intelligence and/or Charisma
• Clerics and Druids rely highly on Wisdom as it is their casting stat, but also benefit from Strength and to a lesser degree Constitution.
• Wizards are all about Intelligence, but are helped a bit by Dexterity as well
• Sorcerers rely on Charisma and also receive a little help from Dexterity

However, not all classes are this simple, or Single Attribute Dependent (SAD). Some classes, unfortunately, are Multiple Attribute Dependent (MAD), which means they have features and functions that key off of different attributes:

• Rangers rely on Wisdom for their casting, but need Dexterity heavily for combat and benefit from Constitution and/or Strength.
• Paladins are equally troublesome, as they need Strength (or possibly Dexterity) for their melee combat while using Charisma for their casting and paladin abilities. (PF actually fixed the worst of this -- the 3.0/3.5 Paladin used Wisdom for casting, which was terribad as it meant the character had to have 3 good attribute scores at a minimum.)

Furthermore, some character classes can afford to "dump", or take a penalty in, certain stats:

• Fighters can generally dump Charisma, as they aren't expected to negotiate their way out of situations
• Wizards often dump Strength, as they aren't getting into frontline combat
• Clerics and druids will frequently dump Dexterity, especially if they aren't using ranged weapons or have Zen Archery -- that feat also allows for SAD archer Rangers, relying entirely on Wisdom.

Overall, this means that P1 and especially P5 must be more careful with how they build their characters. P5 is going to have to go with a fully SAD class with an easy dump stat, for instance -- if they're dead-set on a Paladin or two-weapon fighting (aka dual wielding) Ranger, I'd let them reroll because it's not compatible with their stats array. However, that array would make a reasonable Fighter, Cleric, or Druid.

P1 is a bit more interesting, because they have a double dump on their hands -- most classes only have one designated "dump stat". It is still possible to be effective with two stats dumped, even as severely as that array dumps them, but it takes care to avoid backing yourself into an unexpected corner by dumping the wrong stat. (Dumping Intelligence, Constitution, or even Charisma can have unexpected side effects in certain games -- it may be the case that characters with a low Intelligence will not be able to speak properly, low-hit-point/squishy characters will have trouble surviving first level, or a particularly uncharismatic character will be run out of town before the adventure can get off the ground.)

_{Footnote: these lists are incomplete -- I don't have experience with all the 3.x or Pathfinder core classes}

Bonus: Racial bonuses shouldn't be used to "fill holes"

As to P1's racial attribute bonus? It should go to the dependent attribute for P1's build, not to fill a "hole" in their attribute scores -- you're basically always better off putting it in the strong suit, especially with a +2 bonus because that translates into an unconditional +1 to the derived modifier.