The "free drink" is the last thing left to figure out.
I had to deal with this exact thing when I was doing tours. There were two "tricks" to it:
1. Come in during a slow time of day. They pay their bartenders to be there the whole time. So if you can bring patrons in during a dead time of day, they will be MUCH happier to see you!
2. Have a special short standard drink you offer for free. Give them a tumbler of beer (as opposed to a pint) for free. Make it something simple, cheap, and local. Half the patrons will go for the freebie, and others will "upgrade" and buy their own drink instead. This gives the bar more income. Have a fixed price you pay for the shortie... like $1 each. Also, you don't want drunk people on your tour... so giving them a shortie drink helps keep them from getting too trashed during the tour. (My tour used to stop at several bars, so this could be a problem. Pound a shot at one bar; pound a shot at the second; pound a shot at the third... and all hell breaks loose.)
It's also good form to have some cool info about the space/pub/club/building you are bringing people to. Tell them about the place. Get them interested in the architecture or history of the establishment.
After all, they are tourists, and don't know any place else in town. So if you make THIS place seem interesting, then they are more likely to come back AFTER the tour and buy more drinks, food, strippers, whatever.
Finally, get to know the people behind the bar. When you bring tours in, always go to the same part of the room to give them the story/spiel, away from the bar. As you walk in, make eye contact with the bartender, say hi, and direct your crowd to The Story Spot. This way the bartender knows who you are, and what is going on, and doesn't have to deal with you until you come to them.
Once story-time is over, walk your group to the bar, introduce them to the bartender, and tell them they get to try the local FancyLocalBrewName beer for free, or can buy their own drink. Remind them to tip the bartender.
This is you showing respect to the business, the business owner, and their staff. That way they will always be happy to see you come back in.