How to get sims out of your house?

Teens can live alone. Just go into manage household and remove the parents into a new household or existing one. You’d have to still wait for the parents to get off the lot, but when the parties done you can add them back into the main household.
You can also do testingcheats and remove sims from household, it’s quicker. When you want to add them back have the teen invite them somehow here and add to family cheat

I can think of two ways to have them temporarily leave the house.

1. You can travel to a community lot with the parents and leave the teen at home. Once the community lot is loaded, click on the face of the teen in your household at the bottom of the screen. (like when you tell Sims to work hard) You should see an option to call over the teen but also an option to take over control of the teen. This brings you back to the home lot but leaves the parents at community lot. I don't know exactly how long they will stay at the community lot but I think it will be several hours

2. Send the parents on a vacation. You can pick one night to stay and pretend they take a weekend citytrip or something like that. Play the teen while the parents are on vacation.

Another way to make sure family members stay out of sight is to make them pose. It will freeze them to a pose, and you can make sure it is done somewhere on the lot that you will not use for the party. It requires that you have the poseplayer mod though... Aaaannddd... you might need to keep an eye on their needs, or cheat that needs are frozen during the party (click mailbox, I think that is a BG cheat).

Sending them on vacation sounds more fun though :)

I'm glad I read this all the way through prior to responding. I thought you had Sims on the lot you wanted to have leave. LOL