***** A NON-TEACHER's OPINION *****
(1) What an interesting question. I have checked my books and
am delighted to share the results.
(2) As used in your sentence, it is not considered a prepositional
phrase. Older books call it a focusing adverb, and newer books
call it a focusing subjunct. That is, it focuses attention on some
word or words.
(3) For example, the great Professor George O. Curme in his 1931
masterpiece A Grammar of the English Language gives this
None of them will go; at least John will not/ John at least will not.
(As you can see, the adverb focuses attention on "John.")
(4) Then I found this example in Professor Quirk's famous A
Comprehensive Grammar of the English Language:
At least ten workers reported sick yesterday.
The esteemed professor says that "at least" focuses on the words
Therefore, I guess we would have to say that "at least" in your sentence
focuses on the words "five people."
(5) It would appear that there is no prepositional phrase in your sentence.
Your sentence is basically:
Five people drank wine.
Then if you wish to "focus" attention on those "five people," you can
add the adverb/ subjunct "at least."
P.S. Be careful. Sometimes "at least" is not used the way that it was
used in your sentence. Mr. Michael Swan's extremely useful Practical
English Usage gives these examples of how this "fixed phrase" can be
used in connected conversation:
The car's completely smashed up -- I don't know what we're going to do. At least nobody was hurt.
Another one from Mr. Swan:
Ghosts don't exist. At least, I've never seen one.
And my hero, Professor Curme, gives this example:
He is very poor, at least he has not the wherewithal [resources] to
buy proper clothes for his wife and family. (The professor says that in
this kind of sentence, "at least" can be classified as a conjunction.)
- For Teachers