Actually that is quite incorrect. Anyone can be a Hindu BUT Hinduism is definitely Not a missionary(seeking converts) religion. This tradition has deep roots in Indian history. Even in the ancient days the students go in search of a master.....the master does not go around looking for converts/students. Buddhism on the other hand was a missionary religion (Buddha was not...but later on his students were..hell they even invented 'Buddhism' when Buddha's last words were to "seek your enlightenment yourselves"(meaning that each was to arrive at his own enlightenment according to his research/path...not by aping someone else).) however, even Buddhism was a passive-missionary religion..it never "actively" ran after potential converts.
As Vivekananda once said...Anyone who thinks he is a Hindu, is a Hindu.No more, no less.