( saying ) if you cannot defeat sb or be as successful as they are, then it is more sensible to join them in what they are doing and perhaps get some advantage for yourself by doing so
IF YOU CAN'T BEAT THEM, JOIN THEM
Meaning of IF YOU CAN'T BEAT THEM, JOIN THEM in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005