What is the best method of managing a team in which these members are from a different ethnic group, different grade and different cultures?

The best way to manage them is by indulging those people in kind of work or practice where they can understand each other and they can know about each other for me the best way could be a outing like small get together or other one is sports the best way like cricket or football because they are team spirit sports and by which they can learn to understand each other and they can also learn how to work as a TEAM.

You have to be more specific.

A specific job title cannot include different grades, if it happened then it was a bad decision. Anyway, teaming a group with different cultures and ethnic is very easy as long as you already gave the job description and the Group knows and follows the rules.

Your concern, as a manger, must be patient und understand every person in your team. You have to adjust yourself to their side instead pulling them to you. What I mean is that, you are the manager and the chairman or the board expect from you and think that it is part of your job to understand und know each worker you have and not the opposite. You must have at least the basics knowledge of their culture, religion and behavior in order to know how to take decisions for the sake of the whole group and get more work productivity.

Do not take fast decisions und do not take any disagreement discussion with a worker as personal.

Let them understand your point of view and try to convince them in order to get your productivity target, and if you failed let them know that you are the manager und it is your responsibility to take decision and after all the CEO or Board will  evaluate your work at the end of the year and not theirs.