"He sings with us."
Translation:Anh ấy hát cùng chúng tôi.
Ca has a number of meanings. In this case it means 'to sing' which is homophonous with hát. If you pair the two as ca hát it means singing (in general). Ca can also mean a jug/pitcher or mug of liquid amongst other things like 'elder brother/older male' in words like đại ca (the boss, usually of a gang) and soái ca (handsome individual).
"Ca" also means "ca cải lương," but context is required.
When a Northerner says "ca," they're usually referring to "cải lương" (a kind of Southern opera vs. "hát bội" - kinda like an opera of the North).
When a Southerner says "ca," it can mean any kind of singing; hence, context is needed.