I never joined them so i was wondering if when you join the Dark Brotherhood it affects your reputation to the point you are unable to do something? Does it affect your reputation to the point you can't initiate further dialogue options? Do people hate you?

Please let me know and thanks in advance

No.

Your Dark Brotherhood membership is supposed to be SECRET despite the fact you can wear the outfit anywhere.

Hilariously, some guards figure it out and go, "I know what you are...HAIL SITHIS!"

Which...freaked me out, actually.