The San Francisco Department of Elections does a decent job of reminding people that they need to sign up if they are newcomers or have recently moved. Today, May 19, is the last day to do this in time to vote on June 3. You can even do it online, right now.
Maybe soon we'll have election day voter registration, as ten states and the District of Columbia do now. California enacted this reform in 2012, but awaits certification by the Secretary of State that we have a voter database that can handle it. All experience suggests that reducing registration hurdles increases turnout measurably.
This widget illustrates just a tiny fraction of the information that big data analysis gives political campaigns for shaping their turnout tactics and targets. I imagine the maker put it online as an advertisement.