We do a drill at practice that helps with this sort of situation as well, but I'm not sure if you'd have enough riders for it. We'll ride as a group three abreast, and have the middle row move forward through the outside two lines, with riders alternating sides as they come out to the front. As the drill goes on, the outside lines gradually squeeze in to make it a little tougher for the folks in the middle, so everyone gets comfortable with passing through tight spaces smoothly. By the end we usually have folks squeezing through gaps slightly wider than their handlebars.
That along with the typical bumping drills helped me a lot when I was starting. Also when riding with teammates, we try to sneak up to and bump each other once in a while, or sneak through the gap of two riders having a conversation, just to keep it fresh.