Updated China Visa requirement

Denisse, there is no exact requirement for the amount in your bank account, but you should make sure that it is enough to cover all your expenses when in PRC. Usually, the bank statement should be the one within recent six months. You can provide the copy of US entry permit which may be helpful to your application.
If you want to apply for group tourist L type, you should first join a tourist group and then ask the travel agency to apply for it for you. You four alone cannot get the group tourist L type on your own.