you can create an account for them, and if you want limitate their access to some applications.. or you can go to Disk Utility, select New Image, decide the size of the .dmg file, then in encryption select AES - 128. You'll be asked to choose an username and a password (don't add to the keychain, if you don't want the other people to open it....), and finally you will find that file where you choose to save it... but i will go with the account solution, in particular if your account has administratori privileges