Plymouth Brethren do not employ pastors as leaders of their churches. There is no expressed clergy/laity divide in an effort to affirm the priesthood of all believers, based on 1 Peter 2:9-10.
The leadership that does exist within the Plymouth Brethren takes the form of a group of elders who oversee the care of individual assembly. There are many reasons given for this. They argue that the pattern of scripture does not allow for one man to govern a local church but rather demonstrates that a group of men are appointed from within the church (rather than brought in from outside the church) to oversee the teaching and care of the flock. Since Christ is head of the church, they believe that having one man govern a church body denies the headship of Christ. They often contend that every time scripture mentions an elder, it is as part of a group, demonstrating that no one man is to have total authority over a group of believers.
There is no denominational governance or oversight, typically referred to as each church having autonomy. This can have a very serious affect on church discipline.