に手伝わせよう

let's have [someone] help

→ に

phrase Uncommon #14,030

let's have [someone] help に marks the person who will be made or asked to help; 手伝わせよう is the volitional form of the causative verb 手伝わせる.

JA: 弟に手伝わせよう。

EN: Let's have my younger brother help.