I would suggest the Walmart on 1st Ave S. They can pick up the bike path that goes North on 32nd, and it will take them straight to the route. My group is staying at the Country Inn and Suites. We will either eat breakfast at the hotel and roll up 32nd or we will eat at the HyVee on 1st Ave S and roll out from there.
You could also drop them off at the Hospital SW of town. They could jump on Kenyon road and join people coming from the main campground.