Galaxy groups in the 2dFGRS: the group-finding algorithm and the 2PIGG   catalogue
  The construction of a catalogue of galaxy groups from the 2-degree Field Galaxy Redshift Survey (2dFGRS) is described. Groups are identified by means of a friends-of-friends percolation algorithm which has been thoroughly tested on mock versions of the 2dFGRS generated from cosmological N-body simulations. The tests suggest that the algorithm groups all galaxies that it should be grouping, with an additional 40% of interlopers. About 55% of the ~190000 galaxies considered are placed into groups containing at least two members of which ~29000 are found. Of these, ~7000 contain at least four galaxies, and these groups have a median redshift of 0.11 and a median velocity dispersion of 260km/s. This 2dFGRS Percolation-Inferred Galaxy Group (2PIGG) catalogue represents the largest available homogeneous sample of galaxy groups. It is publicly available on the WWW. 
V. R. Eke, C. M. Baugh, S. Cole, C. S. Frenk, P. Norberg, J. A.   Peacock, I. K. Baldry, J. Bland-Hawthorn, T. Bridges, R. C...