1
general::
Be the principal witness at a wedding, that is, act as best man or maid or matron of honor. For example, Jane asked her sister to stand up with her.
American Heritage Idioms
2
general::
Phrase(s): stand up with someone
to attend someone who is being married. (Usually refers to males.) • I agreed to stand up with my buddy at his wedding. • Tom stood up with Harry when the latter got married.
McGrawhill's American Idioms And Phrasal Verbs