Can one be counted in a minyan if one is present for Minchah/Ma’ariv but has already davened Minchah?
Related question: Should I alter my schedule to daven Minchah with a minyan?
Significant authorities maintain that tefillah betzibbur [=prayer with a minyan] necessitates that there be ten people actually davening [=praying] together, not just ten people present. According to these posekim, if there are six people davening who are joined by another four who have already davened, the former may recite Kaddish and Kedushah, but they do not get the benefit of tefillah betzibbur.
―Rabbi Dr. Ari Zivotofsky, "Tzarich Iyun: Davening with a Minyan", internal footnotes omitted