Summary: We present a model for self-assembly of graphs based on multisets and the formalism of membrane systems. The model deals with aggregates of cells which are defined as undirected graphs where a multiset over a fixed alphabet is assigned to each vertex. The evolution of these aggregates is determined by an application of multiset-based aggregation rules to enlarge the current structure as well as an application of membrane-systems-based communication rules to enable cells to exchange objects alongside the edges of the graph. We compare the generative power of self-assembly membrane systems with and without communication rules, and we characterise properties of the sets of graphs generated by these systems. We also introduce two notions of stability for self-assembly processes that capture the idea of having produced a stable structure. Finally, we investigate self-assembly membrane systems where the alphabet is a singleton.