AUBURN HILLS, MI—Indian automaker Mahindra Group plans to open a 150,000-square-foot assembly plant here.

MUMBAI, India—During the next five years, Indian conglomerate Mahindra Group plans to invest an additional $1 billion in the United States, where it is hoping that its two-wheel electric scooters will win over American millennials. Mahindra has already invested $1 billion in the United States, where it has 3,000 employees.