We present a theoretical approach to devise mul- tiport cavity-excited switched-beam antennas, based on the re-cent concept of metagratings (MGs - sparse arrangements of polarizable particles designed via detailed tailoring of mutual coupling). The discrete nature of the cavity eigenmode basis enables extension of the standard (semianalytical) MG design procedure to allow application of multiple constraints on the same passive composite. Consequently, the static MG is designed to yield a different set of eignemondes on the aperture in response to excitation of different sources (input ports), leading to radiation towards different angles. As verified via commercial solvers, this full-wave-optimization-free scheme yields large-aperture antenna devices with high aperture illumination efficiencies for each of the designated radiation states, providing a simple path to realize dynamic beam steering, especially useful at high frequencies.