public void OnAfterObjectInsert(List<Object__c> insertRecords) {
  List<Object__Share> newShares = new List<Object__Share>();
  List<AccountShare> clientShares = [SELECT Id,AccountId,RowCause,UserOrGroupId FROM AccountShare WHERE AccountId = :insertRecords[0].Client__c];
  for (AccountShare cs : clientShares) {
    if(cs.UserOrGroupId != Userinfo.getUserId()) {
      Object__Share curShare    = new Object__Share();
      curShare.ParentId         = insertRecords[0].id;
      curShare.RowCause         = 'Test__c'; //or Schema.Object__Share.RowCause.Test__c;
      curShare.UserOrGroupId    = cs.UserOrGroupId;
      curShare.AccessLevel      = 'Edit';
      newShares.add(curShare);
    }
  }
  insert newShares;
}