What is the word for attending a party without being invited?
Gatecrashing is the most common term I've heard:
To attend a social event without having been invited, or without having paid.
The term I hear most often is crash:
We're going to crash the party
We're here to crash the party!
Let's crash the party...
My local dictionary offers this:
crash — [informal] enter (a party) without an invitation or permission.
That person would be characterized as an "interloper".