The party started percolating as more guests arrived.

percolating 是动词 percolate 的现在分词形式。percolate 原指液体"渗透、过滤"，引申为"逐渐扩散、活跃起来"的意思。在这里比喻聚会气氛像液体渗透一样逐渐升温、活跃起来。