In TweetCaster, the autofill list does not show everyone you follow
It's a list of users currently in memory pulled from the home timeline.
We can't show everyone you follow, a user could follow thousands of people and we're limited to pulling users 100 at a time up to 15 times in 15 minutes. So basically we couldn't efficiently or effectively make the auto-complete list a comprehensive list of everyone you follow and follows you. To add people you follow that aren't auto-completing, you can use Add User from the overflow and find the user in the list.