In my experience with sit-ins while clubbing, the rule is "follow the singer". Of course this rule can only be followed if the singer is actually in a key. It's actually good ear training to try to figure out where the heck the songs is, in situations like that.

If all of the good advice that you've already received in this thread doesn't help, try to remember that in Church it's God's glory you're there to praise - not your own (although this is faint solace to any player with pride in their performance, it's another lesson in humility).

If you have a singer who just sings flat from time to time, twisting the right ear counter-clockwise usually brings 'em up to pitch. But don't turn too hard or you'll snap a vocal chord.