The Latin root word *clud* and its variants *clus* and *clos* all mean "shut." These roots are the word origin of many English vocabulary words, including ex[*clud*]e, ex[*clus*]ive, and [*clos*]et. When you in[*clud*]e someone, you "shut" him in, thus performing the act of in[*clus*]ion, thereby [*clos*]ing or "shutting" him into your group.