Galaxy groups identified in the spectroscopic sample of galaxies in the field of HE 1104−1805. For each group, the upper plot shows the positions of the candidate member galaxies associated with that group relative to the lens galaxy, with rejected group members represented as red squares and accepted group members represented as green circles. The lens galaxy (star) and group centroid (cross) are also displayed. The R₂₀₀ radius of the group is represented by a solid line, while the dashed circle represents the angular separation cut of the group-finding algorithm in its final iteration. The lower plot shows the observer-frame velocity of individual member galaxies relative to the group centroid as a function of that galaxy’s angular distance from the centroid. Galaxies that passed the iterative algorithm described in Buckley-Geer et al. (2020), Sluse et al. (2017), and Sluse et al. (2019) are shown in green, while trial galaxy members that were cut through the algorithm are shown in red. Horizontal error bars represent the measurement error for each galaxy. The final observer-frame velocity dispersion and angular separation cuts from the group-finding algorithm are presented as dashed and dotted lines, respectively. We also show 1-D histograms and rug plots of the velocity and distance distributions of the member galaxies. The 1-D histograms are produced using a kernel density estimate (KDE) with a bandwidth determined using Scott’s Rule. In the 1D velocity histogram, the dashed blue line shows a Gaussian with a width equal to the observer-frame velocity dispersion of the group.