Leaving Tips
The vast majority of places we use for group meals expect (and deserve) a tip. Typically, this is around the 10-15% mark.
A problem we sometime suffer is those that leave an event early give their money to the event organiser excluding the tip. When the organiser settles the bill they are then saddled with paying that persons tip or at least having an awkward conversation with the waiter why we a short of money.
This is so unfair on the organiser. After organising the event they don't want to end up paying more than everyones else, if anything, they should hope to pay less!
If you object to paying a service charge then take the issue up with the waiter, rather than leaving that confrontation to the night's organiser. Service charges are normally compulsory with groups.
The Golden Rule
If you leave early include a tip in the money you leave with the organiser. If you are unsure of what that should be ask a waiter.
All ensure you have included all your drinks in the payment too.