Should an Avel say Kaddish if they are: 1.) Asked to make a Minyan but already davened this Tefila. 2.) Not needed to make the Minyan but present when 10 are davening and Kaddish is being said. 3.) Arrives at the end of a Shiur when Kaddish Rabbanan is being said, and did not attend the Shiur.
1+2) In the first two cases you can either say a couple Pirkai Tehilim or daven the end of the davening with the Minyan. 3) If you did not attend the Shiur you can not say Kaddish. Rabbi Yitzchak Grinblat.