Sentence:  Lord Silverbridge made up his mind that as he could not dance with Miss Boncassen he would not dance at all. He was not angry at being rejected, and when he saw her <WSD>stand up with</WSD> Dolly Longstaff he felt no jealousy. 
Meaning of the word marked with <WSD> tags:  To begin to dance with (someone); to dance with (someone).