How about allowing Aid Another but making the DC equal to the "EASY" test at the given level, and setting a cap at 3 players aiding the Lead roller. With each Aid giving a +1. Might be a bit arbritrary, but 3 sounds like a fair number. To many Cooks in a Kitchen ruins a Cake so to speak.
The thing about aid another is its not the bonus that makes it so strong, its the fact that a person with a low skill can take themselves out of the skill challenge.
At the most powerful, a team of 5 with one guy having a very high skill, every other player "drops out" effectively and the one guy makes all the rolls. This actually has a tremendous impact on a party's win rate. Even with only letting 3 people aid, you still have that huge effect.