Campaigns can register voters — at least in the states that I’m aware of. Churches can do it. People going door-to-door can register voters. It’s polling places that can’t have any electioneering nearby. Maybe that’s what you’re thinking of?
The other day there was a thread on here where a woman was registering voters in Nevada and was busted by someone for handing out HELLARY pamphlets while registering voters!!! I am just confused as to the difference in what she was doing and this truck registering voters!!!!!