global database.QueryLocator start(database.BatchableContext bc){
    return database.getQueryLocator('Select id,MemberShip__c,Two__c,name From Account');
}

global void execute(database.BatchableContext bc,list<Account> acc){

list<string> se=new list<String>();

        for(Contact c:con){
            if(c.AccountId!=null){
                se.add(c.MemberShip__c);
            }
        }
    for(Account a:[Select id,MemberShip__c,Two__c,name From Account where id in:se]){
            a.MemberShip__c=string.join(se,',');
        acc.add(a);
    }
    update con;
}
global void finish(database.BatchableContext bc){