I'm assuming that you're using the additional people more or less as extras for those numbers that build to a hand-clapping, rollicking finale; "Prepare Ye the Way of the Lord", "Day by Day", etc.

Simply move them in from the wings and down the aisles of the auditorium, hands clapping, and when they're in place they join in singing the final chorus.  One big bow by everyone at the end of the song, then they quickly run off stage and up the aisles and your core of ten resume the show until the next big production number. 

In some of the quieter narrative parts you might move a few (I'd say no more than six) on stage to be part of the crowd listening to Jesus' teaching/parables.  I think that it would be a nice touch, actually, to show the multitudes that Jesus preached to.  I've often thought that just preaching to the disciples sort of limited the impact of the sermons.

I've probably made myself as clear as mud.  In essence I'd say "Use them sparingly", but if it's well done, it could be very effective.