I was invited to an optional meeting by one of my managers to discuss our new website with one of his marketing friends. The meeting is taking place after our normal work hours. It is also at a bar where I am sure that drinks will ensue. Seeing as this is not a mandatory work meeting, do I stay clocked in for the time spent there actually talking about the website?
Normally, you'd book this as overtime if you were actually working.
If it's more of a casual thing and your attendance isn't strictly necessary, then treat it as a social thing, assume that your manager will pay for drinks/food and leave whenever you want to.
Don't forget that this is a networking opportunity. Sometimes the value of getting yourself known is better than getting the overtime payment.