My guess is that the creators of this ad were distracted by the picture of the large-breasted woman covered (snicker) by the strand of red dental floss at the right. (Sorry, I cut that part out to keep a G rating.)
Then again, this is an ad for a bar. You don’t suppose … No, of course not.
On a more serious note, let’s look at why someone made this mistake in subject-verb agreement:
“First” sounds like a singular noun, and indeed it can be, as in “The first to arrive gets a prize.” In this case, though, “first” is an adjective. (That’s the more common usage.) So “first” modifies “100” and “100” modifies the implied noun “people.”
The agreement problem is fairly obvious when you look at it that way: “The first 100 people … receives”? Nah. Not unless you’ve been at the bar way too long.
Let’s hope that next time, the ad makers keep their eyes on the “I” and on the subject-agreement instead of on the, um, dental floss.