Metastable electronic states, ubiquitous in electron-molecule scattering and in ionization processes, still pose a serious challenge to theory. We suggest to make use of electron propagators and present a many-particle theory that facilitates the direct computation of energy and decay width of electronic resonance states in molecular systems. A complex absorbing potential (CAP) is introduced to render resonance wave functions square-integrable and, in this way, representable in standard basis sets. We perform a perturbative analysis, explicitly taking into account all diagrams up to third order, and then employ the algebraic diagrammatic construction (ADC) scheme to derive a numerically efficient and practical method for computing resonance parameters. We call this new method CAP/ADC.