The ability to grow and multiply is a central and indispensable non-equilibrium feature of living matter which leads to striking self-organization phenomena in systems as diverse as bacterial colonies, tissues, tumors or embryos. Here, I will present some of our recent theoretical efforts in understanding how the specific properties of proliferation-induced activities drive collective behavior of such dense cellular aggregates. Using minimal particle-based models, we first investigate how large-scale flows and mechanical stresses caused by growth in confinement interact with the anisotropic shape of particles, such as rod-shaped bacteria, to produce orientational order. This reveals a strong relationship between near-perfect alignment accompanied by an inversion of stress anisotropy for particles with large length-to-width ratios, as well as a sensitive dependence on particle shape. Second, we consider exponentially growing, three-dimensional colonies of motile cells such as tissue spheroids or tumors. By developing statistical measures suited for non-conserved particle numbers, we detect a new kind of mixing transition which is characterized by a diverging mixing time scale despite cellular-scale diffusive motion of individual cells. If time permits, I will briefly outline a volume-conserving analog of this expanding system where similar considerations uncover universal scaling behavior, and discuss connections to other kinds of activities such as metabolism, gene regulation or cell removal.