Mass Determination of Groups of Galaxies: Effects of the Cosmological Constant

13 pages, 3 figures. Version to appear in New Astronomy. Typing errors corrected in relation (1) and in percentage value in page 5.The spherical infall model first developed by Lemaître and Tolman was modified in order to include the effects of a dark energy term. The resulting velocity-distance relation was evaluated numerically. This equation, when fitted to actual data, permits the simultaneous evaluation of the central mass and of the Hubble parameter. Application of this relation to the Local Group, when the dark energy is modeled by a cosmological constant, yields a total mass for the M31-Milky Way pair of (2.5 +/- 0.7) x 10^12 M_sun, a Hubble parameter H_0 = 74 +/- 4 km s^-1 Mpc^-1 and a 1-D velocity dispersion for the flow of about 39 km s^-1. The zero-velocity and the marginally bound surfaces of the Local Group are at about 1.0 and 2.3 Mpc respectively from the center of mass. A similar analysis for the Virgo cluster yields a mass of (1.10 +/- 0.12) x 10^15 M_sun and H_0 = 65 +/- 9 km s^-1 Mpc^-1. The zero-velocity is located at a distance of 8.6 +/- 0.8 Mpc from the center of the cluster. The predicted peculiar velocity of the Local Group towards Virgo is about 190 kms^-1, in agreement with other estimates. Slightly lower masses are derived if the dark energy is represented by a fluid with an equation of state P = w\epsilon\ with w = -2/3