GROUP splits stars into separate groups for possible use during the PSF-fitting procedure. OLDGROUP (or GROUP with option IGROUP=1) splits stars just on the basis of the distance to the nearest neighbor, and needs a critical separation as input. GROUP (with option IGROUP=0) uses brightness information and information about the PSF as well, and uses a critical brightness ratio as input.
GROUP uses as input an output file from the PHOTOMETRY command. The program will prompt for a critical separation or critical ratio, or it can be specified with the CRIT= keyword. The command will also prompt for an input aperture photometry file, an output group file, and a PSF file (for GROUP IG=0). Alternatively, these files can be specified with the DAOFILES MAG=magfile GRP=grpfile PSF=psffile command.