Simply, use BoxCryptor.. It'll encrypt your files & folders with AES-256 standard (AES-256 is classified by the U.S. Government to protect xe2x80x9cTOP SECRETxe2x80x9d information). The best thing: You can use it on any platform: Android, iOS, Windows, Mac, Linux. So, you can sync your encrypted files with Dropbox, too.
Its free Android app: https://market.android.com/details?id=com.boxcryptor.android (Free version has limitation of Read-only access). You don't even need a rooted device whats not the case of fully free LUKS Manager. If your device is rooted & you also need write access without spending money, go for LUKS Manager, but you'll miss cross-platform access. So, my recommendation is paid BoxCryptor if you need write access.
With encryption, your files & folders will be safe even if your friends are hacker. With even fastest super computers, breaking AES-256 standard needs decades or even hundreds/thousands years (based on password strength & encryption passphrase).