In Zeevou, each username is unique. That means each username can be registered in Zeevou only once.
Thus, if you try to add the username as an investor or staff and system does not let you, probably the username has been already registered.
NOTE: You may not have registered the username in Zeevou, but another organisation might have. For example, one housekeeper may work for two organisations. In that case, she/he can't use one email address for both organisations.