I don't understand how that works. CoinJoin works by rearranging input such that the output values remains the same. If the inputs values are unknown, how could that be calculated?
Fair enough, but it still works then. Inputs don't need range proofs. The coinjoin system can be used to scramble the inputs (as long as they have the right sighash).
The outputs (plus public fee) can be summed and proved to be equal to the sum of the inputs. There is an additional proof required that all outputs are positive. This prevents people spending 1BTC and getting -5BTC and 6 BTC, and then just ignoring the -5BTC output.