18-200 is a poor choice. It's too slow for good portraits and will be next to worthless indoors. Also, the chances of you needing anything over 100mm (on a 1.5x crop sensor camera) are slim.
If I had to suggest one lens, I'd say something like a 28-75 f/2.8 zoom. If you're not the official photographer, you might be able to get away with the limitations of the 18-200, but it's still not optimal. The long end of the zoom is seductive, but it doesn't get used much at a wedding.
I like the range of the 28-75 f/2.8 because it's closer to my preferred focal length for portraits at the long end (I like to use a 90mm or 100mm lens for portraits on APS-C, or a 135-150mm lens on film). At 75mm, the zoom is work-able. A zoom that only went to 50mm would not be for me, though for someone else a zoom that went wider might be helpful, so think about the suggestion that someone else made above as well.
ZV