If you put them on the roost when it's good and dark, they'll likely stay put. Use a flashlight with a red light, or put red tape over the flashlight. They won't be able to see well enough to get down, and they'll stay where you put them. I would think that after two or three nights, they'll be roosting there on their own. Do try to put them all together, they still take a lot of comfort from snuggling together, and from one another's scent.