Adding Many to many Data

//Links (relates) an account record to a lead record in a manyToMany relationship
Moniker moniker1 = new Moniker();
moniker1.Name = "opportunity";
moniker1.Id = new Guid("0D50E716-2757-DF11-AF09-000C29A10E7B");
Moniker moniker2 = new Moniker();

moniker2.Name = "contact";
moniker2.Id = new Guid("0D6BF93F-9D94-4EC1-970B-5E1B1BFBADA2");
AssociateEntitiesRequest request = new AssociateEntitiesRequest();
request.Moniker1 = moniker1;
request.Moniker2 = moniker2;
request.RelationshipName = "ppp_opportunity_contact";
service.Execute(request);

Source : http://blogs.msdn.com/b/crm/archive/2008/02/14/crm-4-0-relationships-explained.aspx