There is friendly fire, but only under certain conditions. Newcomers, up to 5000 rank-points, cannot harm other players while they are not in PvP/PK mode. Players with 5000+ rank points deal friendly fire, receiving TK for it, WHILE THERE IS TK - NO RANK-POINTS
More details are described in this topic: https://steamcommunity.com/groups/l4d2badblood/discussions/3/737036097948944868/