Since this is in the English section, I guess you're asking how the average English-speaker would pronounce these names. :) It's unusual for us to start any word with an "ng" sound, so we use the closest pronunciation we know.
Ng = "ing". Yes, we would add a short "i" sound there. Please don't cringe.
Nguyen = "NWEE-yen".
A good speaker (in general) would try to learn the proper pronunciation, as Kraut9 and Eliot have suggested above.