This is why many restaurants add an 18-20% tip to the bill for parties exceeding a given size, usually 4 or 6. That is noted on the menu and thus agreed to in advance.

It ends all of the handwringing about large tips. This tip just looked large, but I wonder how much OP would have flinched about a $18 tip had he been alone and spent $90 on the meal rather than $750 for 8 of those meals.