Set Notification Frequency for all members of a Chatter Group

SetChatterGroupMemberNotificationFrequency.cls

final String NEW_FREQ_FOR_ALL_GROUP_MEMBERS = 'P'; //P=>On each post, D=>Daily, W=>Weekly, N=>Never
final String GROUP_ID = 'CHANGE_ME';  //select id,name from CollaborationGroup

final List<CollaborationGroupMember> members = [
    select id, NotificationFrequency
    from CollaborationGroupMember
    where CollaborationGroupID = :GROUP_ID
    and NotificationFrequency != :NEW_FREQ_FOR_ALL_GROUP_MEMBERS
];
if(members != null && members.size() > 0){
    for (CollaborationGroupMember member : members) {
        member.NotificationFrequency = NEW_FREQ_FOR_ALL_GROUP_MEMBERS;
    }
    update members;
}

Hi,
Please guide me to write the similar code in Batch class.
Requirement is "Need to override the Notification Frequency of all members of the given Chatter Group "
Details:
Digest_Group__c is a custom object with 2 fields "Name" and "Notification_Frequency__c"
Group ID ='0F9K00000004M86KAE'

Here is my code:

global class batchFrequencyUpdate implements Database.Batchable {
global Database.QueryLocator start(Database.BatchableContext BC) {
String g= '0F9K00000004M86KAE';
String members='select id,MemberId,NotificationFrequency from CollaborationGroupMember where CollaborationGroupID = :g';
return Database.getQueryLocator(members);
}

global void execute(Database.BatchableContext BC, List<CollaborationGroupMember> scope) {    
    List<CollaborationGroupMember> members =new List<CollaborationGroupMember>();
    Digest_Group__c nFreq =new Digest_Group__c();
    nFreq = [SELECT Id,Name,Notification_Frequency__c from Digest_Group__c limit 1];
    if(scope != null && scope.size() > 0){        
    for (CollaborationGroupMember mem : scope) {
       if(mem.CollaborationGroup.Name==nFreq.Name){
        mem.NotificationFrequency = nFreq.Notification_Frequency__c;
        members.add(mem);
      }
      } 
        update members;            
    }        
}   

global void finish(Database.BatchableContext BC) {
}

}

Actual Result: Notification frequecy is not updating.
Thanks in advance.