Let $Sigma$ be the Davis complex for a Coxeter system (W,S). The automorphism group G of $Sigma$ is naturally a locally compact group, and a simple combinatorial condition due to Haglund--Paulin determines when G is nondiscrete. The Coxeter group W may be regarded as a uniform lattice in G. We show that many such G also admit a nonuniform lattice $Gamma$, and an infinite family of uniform lattices with covolumes converging to that of $Gamma$. It follows that the set of covolumes of lattices in G is nondiscrete. We also show that the nonuniform lattice $Gamma$ is not finitely generated. Examples of $Sigma$ to which our results apply include buildings and non-buildings, and many complexes of dimension greater than 2. To prove these results, we introduce a new tool, that of group actions on complexes of groups, and use this to construct our lattices as fundamental groups of complexes of groups with universal cover $Sigma$.
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