Since you are not the main photographer nor are your shots really needed, I'd suggest 2 things :
1. Try, try and try. Try high ISO (at worse use it as B/W); Try bounce flash; Try off camera flash; shoot more.
2. Observe if you can how the main photographer goes about his work. From flash usage to crowd control.
Hopefully you will be able to learn from these (ie. yourself and other ppl)
In a church, you will need a really low light capable setup (ie. f2.8 lens and usable ISO1600 camera) if you are not using flash.
A flash will help.
Set the camera settings to something hand holdable and slightly motion stopping (eg. 1/60, 1/90) as well as taking in the ambient (can be a stop under).
Eg. 1/80; f2.8; ISO800
Set your flash to properly expose the couple. Note that you are not trying to light up the whole church. You only need enough light to reach the couple. The higher ISO and lager f-stop will give your flash extra mileage. You should be able to bounce off walls to pull it off (unless the place is Notre Dame or St. Martin in the Fields )
Still too dim, increase ISO, aperture or flash power.
Don't fight for space with the main photographer. You may spoil his rice bowl and your friend's wedding shot.