应改为：If people signed as a deputy, he/she should show clearly on the signature who he/she is on behalf of.
he/she should show clearly on the signature of whom he/she is on behalf不算错,但不太妥当.
下面的第一句最自然,第三句最不自然：
1)Who are you speaking of?
2)Whom are you speaking of?
3)Of whom are you speaking?
参考资料：葛传槼著、上海译文2002《英语惯用法词典》