Results of recent far-infrared magnetospectroscopic studies of a series of GaAs/AlGaAs multiple-quantum-well samples with Si donors -doped both in the 20 nm well centres and in the 60 nm barrier centres are described. In the presence of excess electrons a strong singlet transition of the ions is observed, and one of the two predicted triplet transitions has been identified. At higher electron densities these transitions evolve continuously into blue-shifted singlet-like and triplet-like bound magnetoplasmon transitions. For the two highest-density samples the singlet-like transition exhibits clear discontinuous changes in slope versus field at integral Landau level filling factors (FFs). Plots of the difference in energy between the many-electron singlet-like transition and the singlet transition normalized by the high-field Coulomb energy versus FF, exhibit cusp-like behaviour at FFs 1 and 2 at 4.2 K for the highest density sample, and at 1 and 4/3 at 1.7 K for the lower density sample. The results at integral FFs are compared with recent theoretical calculations. These experiments indicate that controlled doping of a random array of donor ions can be used to probe the nature of many-electron states in confined systems.