Translation:Who are they?
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.
Because shei (who) functions as a question particle because it is obviously a question. Just like if anyone says who, what, where, when, or why in English. It is assumed to be a question.