"having more people": 有更多的人 "is only useful when they work together": 只在他们在一起工作的时候才有用/发挥作用。 putting them together: 有更多的人只在他们在一起工作的时候才有用/发挥作用。 However, this sounds quite weird to me dude:) In real life, I would rather say "人多只在大家在一起工作的时候才发挥作用。" ——simple and easy to understand! Bon courage!

Having more people is only useful when they work together. Translation: 只有大家同心协力，才能发挥人多的优势。 I used the sentence structure --“只有......才”. 只有 means only. “同心协力”is an idiom，心=heart，力=effort，同/协=together, so it means unite in a concerted heart and effort, which is "work together". "人多” means "more people" “发挥......的优势” means "take advantage of...", so "发挥人多的优势" means " take advantage of more people", which is "having more people is useful". Only when more people unite in a concerted heart and effort can we take advantage of them.

只有一起做事情的时候人多才有用。