From Longman Dictionary of Contemporary Englishthe more the merrierthe more the merrierspokenTAKE PART/BE INVOLVED used to say that you are happy for other people to join you in what you are doing ‘Do you mind if I bring Tony?’ ‘No, of course not. The more the merrier.’ → merry
Examples from the Corpus
the more the merrier• "Do you mind if I bring Tony?" "Nah, the more the merrier."• Now Miller Brewing is joining the ranks of the more the merrier.• This might appeal to investors who figure that when it comes to fund choices, the more the merrier. 