I'm trying to print a birthday invitation, 4 to a letter size page. I'd like crop marks for each. How do I set crop marks without having to draw them by hand? Is there a feature when printing or saving to PDF?
Short answer: No.
Better answer: What you're really trying to do is add cut lines. Crop marks are only used as guides for trimming the edges of a sheet. So if you make a one-up document and export that to PDF with crop marks, you can place the PDF four times in your layout and voila! -- Internal "crop" marks!
If you're sending this to press, rather than printing on a desktop printer, don't make them four up unless your printer asks you to. The imposition software that the printer uses will handle the positioning and crops.