In 49 states and the District of Columbia, it's up to each adult to take the initiative to register to vote. In one state, North Dakota, voters show up at the polls, present ID, and vote.
Many democratic countries, including Australia, Canada and Sweden, automatically enroll voters and keep electoral rolls current. Others, such as the U.K., have more proactive outreach methods. According to Pew Research, 56% of the voting-age population turned out for the November 2016 election in the U.S. In Canada in 2015, the comparable figure was 62%; in the U.K. in 2017, 63%.
Some countries make voting compulsory, driving turnout much higher. In Australia, where registration is virtually automatic and voting is mandatory, turnout in 2019's national elections was 92%.