“Many in body” means that each person gives full play to their unique potential and individuality. “One in mind” means that everyone works together based on faith, sharing the same goal and purpose. This is true unity.
From The Wisdom for Creating Happiness and Peace, part 3, revised edition, p. 159

Daisaku Ikeda

There is no more sacred undertaking than our activities to impart hope to people and build peace in our societies. Please advance with confidence and pride.
>From The Wisdom for Creating Happiness and Peace, part 3, revised edition, p. 384

Daisaku Ikeda

We all have our own problems and sufferings. To surmount them, it’s important to have the determination to do so for the sake of kosen-rufu. If we are suffering from illness, for example, then vowing to overcome that illness and gain the strength and life force we need to freely take action for kosen-rufu and demonstrate the validity of Nichiren Buddhism to others will become a tremendous impetus for our recovery. From The Wisdom for Creating Happiness and Peace, part 3, revised edition, p. 52

Daisaku Ikeda