There are commercial editable grids available for Dynamics CRM however in this post I can going to show you can easily create an editable grid using D CRM Editable Grid which is free to use.
You can download the solution from here and once you have installed the solution there are basically two steps to create a grid. The first is to a create configuration record for your grid where you can configure which fields are editable, etc
Once you have created the configuration records you add the D CRM Web html resource file to your form. There is extensive documentation available on the site