Per round, it's easier to figure out how much to tip. Else at the end of the night, if you are running a tab.

15% of the bill would be the usual amount to tip, if you think the service was good: average service is expected to be "good". That means the bar staff attend to you quickly, they clean any mess, clear the empties promptly, get the orders right and are reasonably friendly all the while.

If you're buying for a large group (10+) many would say you should tip more like 20% of the bill. You can just add the 15-20% to a credit card bill, if paying by card.

You can wait until the end of the night to tip (leaving it on the table, even), but to be honest, most people would forget to do it by then. So try to tip as soon as you've bought the drinks.

If you are buying drinks as your friends trickle into the bar, then you could just keep a rough mental tally of what you've spent so far and throw something in the tips jar or hand to the staff after you think you've bought each of your friends one drink.