An Extended Halo-based Group/Cluster finder: application to the DESI
  legacy imaging surveys DR8
  We extend the halo-based group finder developed by \citet[][]{Yang2005a} to
use data {\it simultaneously} with either photometric or spectroscopic
redshifts. A mock galaxy redshift survey constructed from a high-resolution
N-body simulation is used to evaluate the performance of this extended group
finder. For galaxies with magnitude ${\rm z\le 21}$ and redshift $0<z\le 1.0$
in the DESI legacy imaging surveys (the Legacy Surveys), our group finder
successfully identifies more than 60\% of the members in about $90\%$ of halos
with mass $\ga 10^{12.5}\msunh$. Detected groups with mass $\ga
10^{12.0}\msunh$ have a purity (the fraction of true groups) greater than 90\%.
The halo mass assigned to each group has an uncertainty of about 0.2 dex at the
high mass end $\ga 10^{13.5}\msunh$ and 0.40 dex at the low mass end. Groups
with more than 10 members have a redshift accuracy of $\sim 0.008$. We apply
this group finder to the Legacy Surveys DR8 and find 5.2 Million groups with at
least 3 members. About 387,000 of these groups have at least 10 members. The
resulting catalog containing 3D coordinates, richness, halo masses, and total
group luminosities, is made publicly available.
