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?)