While I'm not sure how they'll do it , mobs should give you "something" after wall , I think you assume skilling up will happen by just attacking/using spells and gain 0.1-0.5 skill (much like FFXI skillup) , but the way I see it much like FFX where you need ??? exp to get 100HP/whatever stat/ability and slowly build up your char. unless they stated spacific char development plan and I missed ... kindly direct me to it.
Sure some want to solo and some want to group , main point is not to destroy one by making 1 way far better then the other. maybe make them depend on each other. ex: grouping give you some bonuses for soloing and the other way around.
I don't think a party will have any major advantage over a solo/duo group in FFXIV in terms of skill ups. Here are a few reasons why I feel that way.
In FFXI, you needed cookie cutter members for optimal XP per hour. You couldn't have more than 6 members without hurting your XP per hour. Prior to level sync, even a few level difference screwed over the party.
Now let's go over your typical Boyada tree skill up party. A bunch of misc jobs including mages who melee while performing their role to a lesser extent. You might have up to 12 people and nobody cares about XP. You receive the same skill ups as you would if the group only had 6. Your job doesn't matter because the purpose is skill ups, not XP.
Frankly, I see no point to give a "skill up" party in FFXIV any advantage except their advantage of having more people. It should be riskier to solo/duo. It's the missions, raids, misc content that you should use groups. However, I think you'll see more low-man groups "skilling up" because it's fun to party together. I just don't believe a party setting will benefit the players any on a skill up level. Again, the only reason you see normal parties in FFXI as a priority is because of the XP per hour quota. Without such a system in place, players will choose to solo/duo/trio more while "leveling" their characters.
My logic for this system is very straight forward. SE has tried for years to balance the jobs with very limited success. Would you be more inclined to invite a BST, PUP, or SMN if their presence didn't affect your XP? Say you have 6 people already, you wouldn't mind taking a BST friend if your XP wasn't reduced. The same goes for limited mage support. You typically only have a RDM, SCH, or WHM and that's it. In FFXIV, you can bring 2 healers if you feel like it. Let's say your guild is doing an event later. You need healers for it so you head out to party. Because there is no negative effect for having 2 WHM's, both are able to skill up without hurting the group as a whole. Again in FFXI, you'd be better off kicking one of the WHM's and grabbing a BRD/COR.
In conclusion, there will be tons of stuff to do in a party setting. I don't believe SE will offer any perks to a party because that would only lead to elitism and cookie cutters again. The only perk you should need is safety of numbers. SE has stressed many times how difficult FFXI was to get a party and how they're addressing FFXIV to be more casual. That doesn't mean you can't party, it just means you won't have an advantage which allows you to fly though the character skill up tree. There isn't 100% proof yet, but if you read between the lines, you'll see SE is hinting at my explanation.