Since I didn’t see any responses that directly answered the question of what do you DO, I’ve prepared a short guide for a generic social gathering. This guide may be inappropriate in some contexts such as a dinner party or event/tv show watching party, etcetera:
Show up
Not at the exact start time, but at a minimum of 15-20 minutes “late”
Bringing an unopened bottle of wine or a 6-pack of beer, or another drink of choice is almost always a classy move.
If you drove, don’t park like an asshole. Consider the neighbors.
If you’re standing on the doorstep and you can hear music, it’s probably safe to just walk in (make sure you’re at the right place!). Otherwise, knock/ring doorbell.
Party!
Get yourself a drink and/or a plate of food if snacks are out
Find friends and say hi! You should probably know at least one other person. How else would you have been invited?
Explore! Hosts expect people in their house so it’s generally okay to look around, admire artwork, investigate the music, go into the backyard, etc. Don’t go anywhere that’s obviously closed off, unlit, or otherwise not a party locale.
Talk and socialize. Meet new people! Ask your friend(s) to introduce you to their friend(s). Lightly eavesdrop on convos for something interesting you can talk about. Listen in general. Ask people what they like to do. Share stories about yourself! Pro tip: the length of your stories should be proportional to how well you know the person you’re telling it to. Just met the person? Suuuuper short stories. “No way! That reminds me of the time my cat was in the bathroom when she got hit by a car! The vet said she was very lucky to have a good friend!” (Confession: I used autocomplete to write that story)
Dance!
Find the host and complement their place, the party, the music, food, whatever. Just be nice. Offer to help if they look at all stressed.
Play party games like beer pong or whatever.
Leave! Say thanks. Take your stuff. Cleanup whatever plates/glasses you’ve used. Don’t drive drunk. Don’t overstay your welcome.
This was probably too much info. I have insomnia. I hope someone reads this.