Very simple! Just like you would show or tell a friend that you like what s/he is doing for you. Say thank you.
Greeters do not accept money or anything of value for their services. If during your greet you happen to take a break in a bar or a cafe, it is of course okay to pay for your greeter’s coffee.
Sometimes visitors bring a small “token of appreciation” from home, like some candy, a little souvenir. Nothing of big value, though,