The association reaction NH4++NH3(+M)-->N2H7+(+M), leading to the proton-bound dimer of ammonia, was studied by the CRESU flow technique over the temperature range 15-170 K, in the bath gases M=He, Ar, and N-2, and over the range of bath gas concentrations (0.5-15)x10(16) molecule cm(-3). The rate coefficients are shown to depend on the temperature, the pressure, and the nature of the bath gas. Theoretical modelling of the reaction involves a combination of ion-molecule capture and unimolecular reaction rate theory. It is shown that the present experiments all correspond to the intermediate falloff regime of the reaction ranging up to the high pressure bimolecular capture limit, whereas earlier experiments between 200 and 350 K were made close to the low pressure termolecular limit. Temperature- and pressure-dependent rate coefficients over wide ranges of conditions are calculated and compared with experimental results from this and earlier work. For the bath gas N-2, under most of the applied conditions, the reaction does not follow the energy transfer mechanism, but proceeds via a radical-complex mechanism. In this case, the reaction rate is determined by the capture of NH3 by NH4+(N-2)(n) complexes. The rate of this process is estimated by modified ion-dipole capture theory.