We investigate whether the spectral-type of a locally dominant (most luminous) galaxy can be used to select sets of galaxies that are physically associated (groups). We assume that passive dominants trace a group-like potential, and SF-dominants a field-like environment. The group sample includes 988 groups selected in the 2dFGRS applying a maximum magnitude difference criterion. We find that the average number of passive galaxies associated to a dominant is larger when the dominant is passive, a result supporting our assumption that galaxy associations around a passive dominant are reliable groups. Finally we show that, to reduce the contamination by unbound galaxy associations (SF-dominant), a >=3 passive-members criterion is more efficient than a >=6 all-members criterion.

3 pages, 2 figures, to appear in ''Outskirts of galaxy clusters: intense life in the suburbs'' ed. A. Diferio, Proceedings of IAU Coll. No. 195, 2004