Apartment lease passed down, breach of contract?

STATE: California.

My apartmentmates and I made a deal with four other people regarding passing down the lease of our apartment. We agreed to pass down the lease of our apartment to these people (which officially would be in September) ONLY IF they agreed to be responsible for finding summer subletters (July/August). It is hard to get a unit in this building, so it was essential to us that they agreed to this (or else we would have passed the lease onto others!). They agreed and did indeed find subletters. However, their subletters recently backed out, and they now claim that it is now our financial responsibility to cover these months despite the fact that they have already signed the new lease for our apartment, so we can't even cancel the entire agreement. Can we sue them for breach of contract?

You can yes...they didn't find the subletters as agreed.