Introducing the concepts of distributed generation (DG) and microgrid (MG) are known as key points, which lead the conventional distribution systems toward novel smart grids. Utilizing DG infrastructures would facilitate local supply of loads. Hence, it is necessary to optimally distribute DG units all over the distribution network. MGs with ability of operating in the grid-connected and islanded modes enhance the system performance. Obviously, it is important to implement an appropriate procedure for optimal construction of MGs in a distribution system. This paper proposes optimized methods to allocate DG units and construct a set of MGs with the aim of improving system performance and consumer side properties. It is also taken into account that the generated power by DG units and the consumed power by loads are stochastic. The considered objective functions for optimal DG placement problem are decreasing system losses and improving voltage profile. On the other hand, MG construction is carried out with the aim of maximizing system load factor and minimizing system load variance. In this paper, an improved version of group search optimization (GSO) algorithm named as dominated groups search optimization (DGSO) algorithm is proposed and implemented in order to solve mentioned optimization problems. The proposed DGSO algorithm facilitates the comprehensive search which results in achieving to better solutions. The related formulation, optimization algorithms and simulation results are presented in this paper. Moreover, the proposed strategy is evaluated in the PG&E 69-bus distribution system and a custom meshed distribution network.