The colloquial short form is the possessive pronoun, without an apostrophe. "Where's the party tonight?" "It's at mine/yours/his/hers/ours/theirs."

When you want to say that you were at someone’s place, can you say... The short form is possible but wouldn’t be used in your example. Whose place was the party at? It was at theirs. I love my boyfriend’s apartment. What’s Tim’s place like? His is disgusting. Did your sister come over for Christmas dinner? No, it was at hers.

you can say -I was at his/her/their place.

John, are they used without apostrophes?

hers, his and theirs are the correct usage