Is it sinful to sit with worshippers who are collectively and loudly chanting remembrances after the Fajr or Asr prayers, if the purpose of sitting is to complete individual remembrances and then leave?

The jurists differed regarding raising the voice in dhikr after prayer. The soundest view is that it is permissible for a need, such as teaching; otherwise, it is disliked (makruh). As for raising it collectively in one voice and persisting in that, it is an innovation (bid'ah). Your sitting for your adhkar without participating with the group is permissible. It is advisable to counsel them to abandon that and clarify that it was not among the actions of the Salaf, and to consider the overall benefit and the conciliation of hearts.