Terahertz absorption in two-dimensional electron systems in an AlGaAs/GaAs heterojunction containing a layer of self-organizing antidots is experimentally studied. A magnetoplasmon mode is observed in a magnetic field. When the electron density at the heterojunction is less than the density of the disorder potential minima induced by the antidots we found the drastic narrowing of the absorption line with magnetic field. We interpret this effect by magnetic field induced localization. The localized plasmon resonances are coupled by the Coulomb electron-electron interactions leading to collective magnetoplasmon excitations.