吗 is only used for yes/no questions

谁 already turns it into a question.

The word 谁 is already a question word meaning "who?", so it would redundant and unnecessary to add 吗 to the end.

*Usually you need 吗 to change a sentence into a question that does not already contain a question word.... Eg: (你吃了, you've eaten) (你吃了吗，have you eaten?) (你吃了什么，what did you eat?)

吗 is used if you are asking a question which requires a yes/no answer. You cannot answer "Who are they?" with a "yes" or "no."

Ma is just a particle, or a classifier, so we might not use it when talking about talking about people first person. I'm not a pro, but I predict it.

I think it's beacuase "who" already means a question

The second one