Yup, everyone has everyone else's sender key. This is more or less equivalent to having a single shared group key, but there are some practical reasons why it'

Yup, everyone has everyone else's sender key. This is more or less equivalent to having a single shared group key, but there are some practical reasons why it's split out. I think one of them is that it makes symmetric key ratcheting (for the purposes of perfect forward secrecy) a little less disastrous in the case of out-of-order or failed message delivery. Not totally sure, though.