I don't know how they do it but it seems easy in practice. You hash the concatenation of chunks of data submitted by participants. Adding more data to the hash

I don't know how they do it but it seems easy in practice. You hash the concatenation of chunks of data submitted by participants. Adding more data to the hash can only make it more random, not less. As long as at least one participant wants the outcome to be random and therefore they submitted actual random bits, the result will be random no matter how unrandom all the other participants' data chunks were.