mining pool hashrate does affect your earnings as the 25btc block still has to be split by how many shares you submitted so for example a block took 100 shares and 1 user submitted all those share they would get 25bitcoin where as if 2 users submitted share they would get 12.5 each
Indeed, but in the case of two users, you would win blocks twice as quickly since the pool's hash is doubled so the overall earnings would be the same.