Firstly I have to emphasize it's just an opinion since it is impossible to be sure and base on any official info:
I think that loot in case of Zushuka/The Welter/Ocyakao etc. does not depend on a players. In my opinion there is the same chance to drop rare item if you kill it solo and in team. If it was be true, that it is easier in team, it would be really easy to loot stuff from The Welter, since it is very often killed by a few players, but usually this boss simply drops nothing special and all team get nothing. Worth to mention that even if boss will give a rare, lucky person will be the only who get something valuable. Rest of team will get only some useless items and only the first winning loot may be valuable.