See the John Denver video.

http://www.youtube.com/watch?v=nc_LyxO3Ncg

Sing one verse and chorus solo to set the tone. Then walk close to or into the audience and ask them to join with you in singing this tribute to the couple. Arrange for two or three cohorts in the audience to back you up. Result: Everyone participates in and blesses this marriage. (Consider handing out copies of the lyrics if needed.)