"QFF Gold means that I can enter any oneworld lounge if I have a oneworld flight that day. Eg. I am at Washington Dulles waiting for a flight with AA. Can I go into the BA Lounge, or am I restricted to the Admiral's Club?"

Correct - although they would need to be leaving from the same terminals, to facilitate security concerns.

"Another question, can I bring a guest into the oneworld partner lounge?"

normally not a problem, but every time I have done it, the guest is on the same flight as me

Dunno wha the situation is as far as guests travelling on different flights, whether they be oneworld or not.

Dave