“They should just take a few pictures and Photoshop all the combinations later. The whole thing could take 30 minutes.”
I just thought of an even better idea. On the response card, they should write something like “If you can’t/don’t want to come, please enclose a recent picture of yourself, preferably smiling” and then just Photoshop everything. That way you can save time and save money on the number of guests also!