If you want a detailed answer check out the recent book I just bought for my shul–At the Amud-(Gordon) Page 58. Actually the book covers most topics of tefilla -especially during aveilus and Kaddish in general– We were having the same problems in our shul so I bought it for future reference and hopefully, less arguments.-
-In short–some poskim say that it should be said together in one place but R. Yehoshua Neuvirth z”L and Rav Chaim Kanievsky ????? write that it is better for those saying Kaddish to stand apart so that each will be heard and answered by a minyan.