Also, even if you don't use it, group.entities.add_faces_from_mesh and group.entities.fill_from_mesh seem to delete everything existent in that group.
Edit: thomthom corrected me, sorry. Also, i wrote group.fill_from_mesh instead of group.ENTITIES.fill_from_mesh.
I'm going to crawl back to my hole now, one post, two mistakes is quite embarrassing. 😢
T