buy them a drink? Your paid in beer, so tipping in kind seems reasonable? Tipping bar staff in the UK is pretty rare - if you're sat at the bar all night you might offer to get them a drink.
Occasionally we get a gig which pays a LITTLE less, and has a round of drinks thrown in, but on the very rare occasion we've had an free bar all night it's been a waste of time - One guitarist doesn't drink, I'll only have one, the rest of the band MAYBE two - 7 drinks max. Normally we all drink coke. We've all got a gig to play, and have to drive home. Now if they'd let us play one night, and have free drinks the NEXT night, we could get our moneys worth.
Ian