Hominidae is to hominid as Poaceae is to what?
Since Poaceae is the plural of Poacea in Latin - like aquae is the plural of aqua - I would say a Poacea, but I'm not a native English speaker so I don't know if this form would be acceptable in English. My guess is that you will have to use the long form a member of Poaceae. In languages like Italian and French it's easier because the Latin nomenclature and grammar are translated. So in Italian you will say una Liliacea for the singular and due Liliacee for the plural, while in French it would be une Liliacée and deux Liliacées respectively.