  A grand canonical system of hard-core bosons, subject to thermal
fluctuations, is studied on a lattice. Starting from the slave-boson
representation with fields for occupied and unoccupied sites, an effective
field theory is derived in which a complex field corresponds with the order
parameter of the condensate and a real field with the total density of bosons.
Near the boundary between the normal and the superfluid phase we obtain the
Ginzburg-Landau functional for the superfluid order parameter. A mean-field
calculation shows that the critical temperature $T_c$ increases with increasing
density up to a maximum and decreases with further increasing density.
