The one-male reproductive strategy implies that maturing males are temporarily excluded from reproduction. In gorillas, these excluded males live either solitarily or in nonbreeding groups (NBGs) that are devoid of adult females. The dynamics of NBGs are not well known. In this study, which was conducted on a gorilla population (Gorilla gorilla gorilla) of 377 individuals that visited the Lokoué clearing in the Republic of Congo, we detail how the NBGs formed, and analyze their dynamics according to age-sex classes, the relatedness of members, and the origin and destination of transferring individuals. We discuss the potential benefits gained by individuals living in these groups. The NBGs included mainly immature males, most of which appeared to have migrated voluntarily from their natal groups. Some individuals (including juvenile females) came from disbanded breeding groups (BGs). Migrants preferentially joined NBGs that included a silverback male. Their dispersal patterns were not determined by their degree of relatedness, but they tended to associate with related silverbacks. In this way, the migrants could enhance their protection against predators and gain experience with different environmental conditions. By tolerating and protecting offspring, aging silverbacks could enhance their inclusive fitness. Finally, young and healthy silverbacks could increase their likelihood of forming a future BG when unrelated females joined them.