This might help anyone that doesn't have success with Whatachamp's solution on the first try.
whatachamp's solution worked perfect for my "coach" role, but not for my "sub-admin" role. All users with the role "sub-admin" would not show in the list even after I completed whatachamp's solution.
I finally found the cause of this was due to the User Level being set to 0. If I changed the User Level to any number between 1-10, whatachamp's solution works. As a side note, once the User Level has been changed, any users created with the custom role thereafter will always show in the drop down list.
I'm sure there are many different ways to change a role's User Level, though I used this plugin to switch it: Capability Manager Enhanced
Does anyone have a solution to change the user level that does not require a plugin?