"Happy birthday"

Unless they have BF there, then they have to make an excuse

Next move-hand down the shorts and then off. She will either take her right there or they will leave the party quick

I'm guessing they are closet lovers and they were at th party and just went out back to let out the frustrations they had inside. A little wink here and a touch there and boom, they are in the back yard ready to have sex. The one on the left really wants the other one. Notice her bra is almost off and the top was pulled down quick. That's just a guess, but sounds good.

That had to have been some party.