I didn't really think you needed an invitation for open house parties and that sorta stuff. My friends and I usually arrive at house parties uninvited all the time and it's not much of a big deal.
Find out when the party stars, than arrive like an hour late or so when people start having their buzzes and the like, where they'll be a friendly party vibe.
Find out when the party stars, than arrive like an hour late or so when people start having their buzzes and the like, where they'll be a friendly party vibe.