My Macbook’s hard drive is encrypted using FileVault 2. Will adding a new user with a different password break the encryption?
FileVault 2 will encrypt the boot volume and allow you to unlock that volume based upon existing user accounts.

Adding a new user account on OS X El Capitan (10.11.6) will automatically add that account to FileVault 2's list of enabled users.  So no, you won't revert your existing setup or remove an existing account's ability to unlock an encrypted volume.

The built-in fdesetup account is also very useful (requires sudo privileges):


 list       [-extended] [--offline] [-verbose]
            List enabled users, or locked volumes.

 enable     [[[-user username ...] [-usertoadd added_username ...]] |
            [-inputplist]] [-outputplist] [-prompt] [-forcerestart]
            [-authrestart] [-keychain | [-certificate path_to_cer_file]]
            [[-defer file_path] [-forceatlogin max_cancel_attempts]
            [-dontaskatlogout]] [-norecoverykey] [-verbose]
            Enables FileVault.  This command will fail if no recovery par-
            tition was found on your disk.