Not sure exactly who invented it, but the word is originally from the Latin for "to ask". The symbol itself, "?", is thought to be derived from the first and last letters of "quaestio".
The question mark is a formalised letter �Q', which monks used in the Middle Ages as an abbreviation of �Quaestio', the Latin for �question'. It was followed by a full stop/period to indicate that it was an abbreviation. Over time, the two combined into the form we are familiar with today. So, the answer to your 'who' query is...monks.