About 5 premade games

YO, i saw alot of players premade in ranked ,they won alots when 3 4 5 premades ,but they lost alots when solo Q.... then i question to my friends "why", they said : some times 4 boys(high diamond master challanger) with 1 girl, then they will easy win till diamond 3,but dat lady,still bronze skill... Or some times 3 4 5 elo boosters premade for jobs... please Riot tell me,shoujld i join them?? help you kill this game?? happy new year ^_^