"a lot of guests" is one complete idea. It's the same as "many dogs" or "a few cats". It's a simple sentence that places emphasis on the amount or quantity of the subject. (Guests, in this case).Originally Posted by Snow Lau
"a lot of the guests" is an incomplete thought and requires something else to make sense. A lot of the guests .. what? You could say a lot of the guests left, a lot of the guests are wearing ties, etc. This sentence is no longer placing emphasis on the amount of quantity of the subject, but rather on what the subject(s) is(are) doing.