Hinduism spread in the world with the contributions of great hindu preachers. The exact year of origin of hinduism are not known however most of the historians consider 8th century as the year when hinduism started spreading. The Aryans (invaders) arrived from the Baltic sea which came up with their indian religion and beliefs. Aryans brought lots of changes in the way of living of people and brought caste system in the society. It gave emergence to other religions as well such as Budhism however hinduism continued to evolve uptill now and is one of the biggest religion in the world.
Hinduism is largely practised in india.so it spreads when indians migrate to every nook n corner of the world esp.the developed countries. Hinduism doesnt force people of other beliefs to convert or if a member of another religion marries a hindu.
Hinduism will spread with the holy spritual, religious behaviour of an HINDU