Hi

I hope it will help you

Actulay in my scenario am sending the email for contact who contact role is primary

public class Sending_Email
{
public List<Opportunity> Opp=new List<Opportunity> ();
public List<contact> con=new List<contact>();
public List<OpportunityContactRole> ocr = new List<OpportunityContactRole>();
public set<id> ids=new set<id>();
public Opportunity opport;
public List<id> ocrids=new List<id>();

public void se(){
// getting the list of all opportunity where stage name is contracted performance
Opp=[select id,Name,StageName,Accountid from Opportunity where StageName='Contracted Performance' and ];
system.debug('#####'+opp);
for(Opportunity op:opp)
{
ids.add(op.id); //adding all the list opportunity in a set to get the ids
}
// getting the contract role where primary key is checked for all opprotunity
ocr=[select ContactId,Id,OpportunityId,IsPrimary from OpportunityContactRole where IsPrimary=true and OpportunityId in :ids];
system.debug('@@@@@@@@@@@'+ocr);
for(OpportunityContactRole oc:ocr)
{
system.debug('*********@@@@@@@');
ocrids.add(oc.ContactId);   // getting the all contacts of emails,where the primary key is true
system.debug('*********@@@@@@@'+ocrids);
}

        Messaging.MassEmailMessage mail = new Messaging.MassEmailMessage();
        mail.setTargetObjectIds(ocrids);  // assign the all emails
      //mail.setReplyTo('exemple@exemple.ex');
      EmailTemplate et= [SELECT id FROM EmailTemplate WHERE developerName = 'Emails_Before_Event'];
        mail.setSenderDisplayName('Incognito Artists'); // displaying the senders name
        mail.setTemplateId(et.id);  // adding the templete id,here we are taken subject,body.
        Messaging.sendEmail(new Messaging.MassEmailMessage[] { mail });
        
 }
      
}