[SalesForce] Insert failed. First exception on row 0; first error: INVALID_CROSS_REFERENCE_KEY, invalid cross reference id: []

I am trying to add users to the chatter groups based on region field on User object using trigger and apex class.

When ever I am creating or updating a user it is giving Invalid Cross reference key Error

I hardcoded the Id's but still same error !!

Class:

@future
public static void groups(Set<Id> userIds)
{
    List<User> users=[select id,UserTheater__c,Name,Username from User where id in :userIds];
    List<String> groups=new List<String>{'A','B','C'};


    List<CollaborationGroup> chatGps=[select id, Name from CollaborationGroup where name in :groups];

    List<CollaborationGroupMember> chatGpMem=new List<CollaborationGroupMember>();
    for (User user : users)
    {
        for (CollaborationGroup chatterGroup : chatGps)
    {

    CollaborationGroupMember c =  new CollaborationGroupMember();
    if(user.reg__c == 'TL' && chatterGroup.name == 'A' ){   
        c.CollaborationGroupId=chatterGroup.id;
        c.MemberId = user.Id;
    }
    else if(user.reg__c == 'DL' && chatterGroup.name == 'B' ){
        c.CollaborationGroupId=chatterGroup.id;
        c.MemberId = user.Id;
    }

    else if(user.reg__c == 'ML' && chatterGroup.name == 'C' ){
        c.CollaborationGroupId=chatterGroup.id;
        c.MemberId = user.Id;
    }
    chatGpMem.add(c);

    insert chatGpMem;

}

Trigger:

trigger grpUser on User (after insert,after Update) 
{
  clsName.Groups(trigger.newMap.keySet());
}