stand up with (one)
To be one's best man or maid of honor at one's wedding My brother was a bit hurt that I asked my friend to stand up with me instead. Jenny wants me to stand up with her at her wedding this summer, but I don't think I'll be able to do it.
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.
stand up with
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.
stand up with
To act as best man or maid of honor for (the groom or bride) at a wedding.