import com.atlassian.jira.component.ComponentAccessor
// the group you want to remove users from
final String groupName = "Group_A"
// user names of the users to remove
final List usersToRemove = ["user_A", "user_B"]
def userUtil = ComponentAccessor.userUtil
def userManager = ComponentAccessor.userManager
def group = ComponentAccessor.groupManager.getGroup(groupName)
assert group : "Could not find group with name $groupName"
usersToRemove.each {
def userToRemove = userManager.getUserByName(it)
userUtil.removeUserFromGroup(group, userToRemove)
}