Probably not. I think what you're looking for is one of the following: "There are going to be over 100 people attending the party." "There are going to be over 100 people showing up at the party". Now, I said "probably not". That's because the word "party" can sometimes be used to describe a sales event. These events are usually for products such as Tupperware or Avon products. So, it is possible, although unlikely, that someone could organize a big "party" with lots of different vendors attending. If there were over 100 of these vendors, then your sentence makes sense. (However, any "party" that big would probably be called a "convention", "expo (exposition)", or simply "sale" instead.)

You don't need "showing ". Just say, "There are going to be over 100 people at the party."

" There are going to be over 100 people showing at the party " This is perfectly correct! There is an optional "up" if you like.

Yes it does but you're missing the word "up". There are going to be 100 people showing up at the party.