Methylammonium (MA)- and formamidinium (FA)-based organic-inorganic lead halide perovskites provide outstanding performance as photovoltaic materials, due to their versatility of fabrication and their power conversion efficiencies reaching over 22%. The proposition of guanidinium (GUA)-doped perovskite materials generated considerable interest due to their potential to increase carrier lifetimes and open circuit voltages as compared to pure MAPbI3. However, simple size considerations based on the Goldshmidt tolerance factor suggest that guanidinium is too big to replace methylammonium as an A cation in the APbI3 perovskite lattice, and its effect was thus ascribed to passivation of surface trap states at grain boundaries. As guanidinium was not thought to incorporate into the MAPbI3 lattice, interest waned since it appeared unlikely that it could be used to modify the intrinsic perovskite properties. Here, using solid-state NMR, we show that not only is GUA directly incorporated into the MAPbI3 and FAPbI3 lattices, forming pure GUAxMA1-xPbI3 or GUAxFA1-xPbI3 phases, but we also find that it reorients on the picosecond timescale within the perovskite lattice, which explains its superior charge carrier stabilisation capacity. Our findings establish a fundamental link between charge carrier lifetimes observed in photovoltaic perovskites and the A cation structure in ABX3 type metal halide perovskites.