In order to work together in a ministry context, Christians need to agree about the gospel. How are we saved? What are we saved from? What is the basis of our right standing before God? If Christians disagree about the gospel itself, no true cooperation in matters of ministry is possible.
Beyond that, the amount of theological agreement Christians should have in order to work together will vary depending on what work they intend to do. In order to be in the same local church, Christians will need to agree about issues like the proper subjects of baptism and the biblical form of church government.
At the same time, there are many ways Christians who don’t belong to the same church can practically work together for the good of the broader church, including partnering for evangelistic work, preaching and teaching in one another’s churches, and so on. Such work outside the local church will require less agreement, particularly in peripheral matters. For instance, there is no reason a Baptist and Presbyterian cannot do evangelism together.