I usually say "Bicycle behind you" before I am actually to close so they can make the decision as to which way they want to move, right or left. I have had people move left when I have said "on your left" so now I just give them the option with enough notice so I am prepared to go right or left around them. I always say "Thank You" after passing them, they seem to appreciate that. You will always find the group who refuses to budge even if you ask them to move 3 or more times, then when they finally decide to move I give them a dirty look as I pass them just to let them know I didn't appreciate them not responding to my simple request.