I've heard both versions, usually in similar contexts. Which one is correct or more correct — or more prevalent — in the USA?
He: This deal ends at 7 p.m.
She: Sucks, I am late to the party.
"Late to" implies that you are present, but late enough that the "party" is already well underway. "Late for" implies that you haven't arrived yet (or have just arrived) and the "party" has started.
The respective connotations are that if you are "late to" a party, the best food and drink is gone, conversation groups and dance partners have formed which are harder to chisel into, people have already had a few drinks, and generally you have missed out on having the best time. This holds when used figuratively; a person who shows up to a sale after the item they wants is gone or the deadline has passed, or in business terms if a "late mover" enters a product market that is already in decline, they have "missed out" on getting the best deal or a good share of the revenue.
Being "late for the party" is less commonly used figuratively, but is often heard in context of a new person joining a group just before (or after) the group embarks on something: "Am I late for the party?". The connotation is that the "party" (the event in which the person wanted to participate) may or may not have begun; the speaker is asking which.
"Late to" suggests that you are not going to make it to the party on time. This also applies to other events:
The focus here is that whenever you arrive, it won't be on time.
"Late for" encompasses the "late to" definition but also includes the idea that you are now at the proper location but it is so late that you missed the event entirely:
To say the earlier meaning:
Notice the switch from "I will be late to" into "I am late for". Technically, this is more accurate but real usage suggests that each phrase is completely interchangeable.
I believe the question here is being directed to the figurative use of the phrase. Being "late to the party" means most people became aware of something before you did, or adopted something before you did. For example:
In this case, it's always "to" the party, not "for."