Britney's former assistant, Alli, is definitely getting used to the Spears lifestyle -- hitting up a celeb-packed party over the weekend -- without Britney!

TMZ cameras caught Ms. Sims at the Polaroid House for Wilmer Valderrama's party on Saturday, no Spears in sight. When asked where the Britster was, Alli replied, "She's at home right now," adding Brit wouldn't be coming to the party.

Clearly learning a thing or two about avoiding personal questions from her boss, all Alli would say about Brit's latest rumored conquest, Criss Angel, was "he's a great magician."