When my parents invite their friends to a restaurant, my parents usually pay. However, the friends would always offer to pay (it's considered polite to do this in Vietnamese culture) and my parents would turn it down.

I agree with Daniel, though. In American culture, a group of friends usually split the bill (doesn't matter who invited who). Most guys I know will pay on the first date. Some will insist on always paying for the girl. Some girls like this. Personally, I find it insulting if he never lets me pay, but that's just me.

I agree with everyone's answer.

I am particulary impressed with Quynh's response which shows that she is fair-minded, independent and considerate.

Good answers everyone...

If you are a real gentleman, you will always pay the bill.

In japan, senior people will often cover other's bill when going out for dinners. It's rule but nobody won't say something about the rule. I don't know what this expression is correct but It's just a dark rule, unvisible rule. 

But yes, one absolute fact in India is that if you invite somebody or somebody is your guest. You will ALWAYS pay.