How to add a second instrument in the middle of the score

Hi all, I'm trying to arrange an orchestral piece for a group that I'm performing in, and there's a ton of instruments. The first 70% of the piece is orchestral, then the last 30% includes saxophones and split trumpet parts. The saxophones don't play at all for the whole piece until then, which is about 230+ measures in. How can I hide those instruments in the score until they come in without hiding the other important instruments during small amount of rests?