Man pages matching name mpi_group_union in section 3:

MPI_Group_union.3.en	debian 10 / lam-mpidoc
MPI_Group_union.3.en	debian 10 / mpich-doc
MPI_Group_union.3.en	debian 11 / lam-mpidoc
MPI_Group_union.3.en	debian 12 / lam-mpidoc
MPI_Group_union.3.en	debian 12 / mpich-doc