I have a simple question, is it possible for a group of enemies to actively hinder a skill challenge?
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maybe in a social setting based challenge: the enemy NPCs are running their own skill challenge with DC's based on the player rolls to counteract some of the successes?
My response to this idea is heavily influenced by LostSoul's ideas about running skill challenges (and I strongly recommend looking at his Keep on the Shadowfell for ideas on running skill challenges, if you haven't already).
The general logic of a skill challenge is that only the players get to roll. I don't think that having the NPCs make skill rolls adds anything - for example, even if the NPCs succeed at their "skill challenge", there is no way within the game rules to force the players to acknowledge their success; and if the NPCs fail their rolls then what happens - are you going to give your players' successes for free? (In this respect skill challenges are quite different from combat, where both PCs and NPCs have hit points to lose, and the rules of the game to require players to acknowledge NPCs' success in combat.)
So enemies can hinder a skill challenge, but not by making rolls. Rather, as a GM you should use them and their responses as part of your repertoire for driving your players through the challenge. This can also be useful for getting all the players involved, even those whose skills are not optimised for the challenge.
So, for example, the enemy NPC with a good Bluff score says to the king "Look at that warrior - an uncouth mercenary! Would you really trust your kingdom in the hands of one like that?" You tell your players that the king nods in agreement. Now the player of the fighter has a choice. S/he can do nothing - in which case it clocks on as a failure for the party, and they are closer to losing the challenge. Or s/he can make a Diplomacy roll to have her PC defend herself, trying for a success or risking a failure. Or a more clever player might come up with something different that better plays to his/her PC's strengths: perhaps using Streetwise to get scuttlebut on the NPC so that the latter retracts the criticisms, or finding a way in which his/her PC can help another of the PCs rebut the criticism (eg player makes a successful Athletics roll, and another player makes a Diplomacy check with a +2 bonus to explain "Well, he may be uncouth, but he makes an invaluable ally in these dangerous times!").