Anudeep thanks for replying.
I did not explain it well. I am including my code to assist.
The problem I cannot solve, is this.
This batch executes on contact members of a campaign. It is triggered by a logic bolean on the campign record.
When the box is checked but the batch processed date is null, then the batch runs.
When the batch is finished, I need to populate the processed date on the campaign. 
I am successfully creating the batch, and sending the email when it is done.
What I am not able to do is populate the campaign record field processed date. 

So what it tried was to populate a string cId with the campaign id from the scope. That way in the finish, I could do an updated on that record
But whenever I try to extract the CampaignId from scope and assign it to Cid I get an error saying invalid list error. 
Once I get cId populated, I would just do a lookup on the campaign record, and update the processed date.
That value is stored in loop variable cm.CampaignId or in the scope CampaignId field

//Batch Class implemented on database.Batchable for an sobject defined in the start method

Global class batchCM implements Database.Batchable<SObject> {
    global string[] email = new String[] {'tj@life.com'};
    global String[] cId = new String[]{''};

/*  database.Batchable requires three methods, Start, Execute and Finish. This is the start method
    and it defines the scope of a batch based on a SOQL Query*/

    global Database.QueryLocator start(Database.BatchableContext BC){
    return Database.getQueryLocator('SELECT Id, CampaignId, ContactId, Campaign.Form_Process__c, Campaign.Form_Process_Date__c FROM CampaignMember Where Campaign.Form_Process__c = True AND Campaign.Form_Process_Date__c = null');
}

/*  The second required database.Batchable method is execute, where the work is done
    in this case we are for looping through the scope records and creating new community form
    records for each scope campaign member record*/

    global void execute(Database.BatchableContext BC,List<CampaignMember> scope){
    List<Community_Form__c> newFormList = new List<Community_Form__c>();
    for(CampaignMember cm : scope){
    Community_Form__c cform = new Community_Form__c(CM_Id__c=cm.Id,
                                                    Contact__c=cm.ContactId,
                                                    Campaign__c=cm.CampaignId,
                                                    Status__c='New',
                                                    Date__c=system.today(),
                                                    Type__c='CM',
                                                    Active__c=true,
                                                    Comment__c='Created from Apex Batch');
    newFormList.add(cform);
    }
    if(newFormList != null && newFormList.size()>0) {
        Database.insert(newFormList);
// left off here did not work        cId=String.valueOf(cm.CampaignId);    
    }
}

/*  Thhe final method is the finsh mehtod that should be sending an email to me saysing the batch is done
    but this is not working at the momeent
    To Do 
    Replace formulas on form object to contact and replace with static fileds updted in the batch
    Add the update on the campaign to populate the procesed date
    look at the flow to see if we can switch between smaller campaigns to run from the flow vs batch on larger campaigns
    */

global void Finish(Database.BatchableContext BC){

    Messaging.SingleEmailMessage mail=New Messaging.SingleEmailMessage();
    AsyncApexJob a =[select a.CompletedDate,a.JobItemsProcessed,a.NumberOfErrors,a.TotalJobItems, a.Status From AsyncApexJob a where id=:BC.getJobId()];
    mail.setToAddresses(email);
    mail.setReplyTo('tj@life.com');
    mail.setSenderDisplayName('Apex Batch Job');
    mail.setSubject('batch processing ' + a.Status);
    mail.setPlainTextBody('Job is done Records processed' + a.TotalJobItems
                        +' Number of errors '+ a.NumberOfErrors
                        +' Status '+a.Status
                        +'Job Items Processed '+ a.JobItemsProcessed
                        +'Completed '+ a.CompletedDate);
    Messaging.sendEmail(new Messaging.SingleEmailMessage[]{mail});
    }
}