How do you give a guard a drink?

You go and buy bottled water or some refreshing drink from a Mart and go back and talk to the guard. Then you may proceed with whatever you are doing.

For me it was free. I went to Celadon City and took the underground tunnel. When you come into celadon city from Route 7 go past the Pokemon Center and the first that you can enter (the 4 building from the Pokemon center going left) when you enter it go foward and take a left, Then go to the old lady siting at the desk and she will give you some Tea then go all the way past the Pokemon center again (going right) go to the guard that wouldn't let you pass before and she will share the tea with the other guards so you can now pass through any guard post (or whatever their called)

You must give the guard a coffee, I believe. You can get a coffee from someone or somewhere in the game.