group dinners - what are they worth?

As I try to become more social with the locals; last night was another experience. So it was a girl's birthday who is ok but can be a little much to handle from time to time. She invited me to her dinner - Chevy's (American-Mexican national chain). I bolted out of work and made it home, changed and bolted to the restaurant. There I saw 15 mid-single folks, some I knew and some I didn't. I sat at the end of a growing table started to mingle. I overloaded on chips and salsa ... stuff gets me every time! So I went with a side of black beans and corn mix...happy with my selection. Well the end of the meal comes around and where up to about 25 people. I'm sitting there and money is passed my way...now I don't ask to be the collector but the task is handed to me...OH JOY! Now most of you know my feelings about large dinners and form of payment. Suddenly there are about 4 people who say, "gee all I have is a card..." well I say, "SO" and roll my eyes and say put X amount on the card and then leave a tip. Well they weren't comfortable with this and I said, "well next time come prepared, this wasn't a surprise to you." A couple people were shocked with my response. I ask "WHY?" Well the bill finally arrives and they charge 18% tip...no brainier, a few people complained and didn't want to pay so every one that handed me cash and asked for money back I had to say, "are you including tip and tax with your guess?" In the end we had several cheapskates and I ponied up much more then was necessary. In the car I started to wonder...is being socially really worth that much...I caved and said...YES and went to the house for birthday cake and of course it was carrot cake...just my luck.