Add users to a jira group

AddUserstoGroup.groovy

import com.atlassian.jira.component.ComponentAccessor

// the group you want to remove users from
final String groupName = "jira-users"

// user names of the users to remove
final List<String> usersToRemove = ["john.doe","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 userToAdd = userManager.getUserByName(it)
    userUtil.addUserToGroup(group, userToAdd)
}