Click to add a layout. Enter in the Layout Name, select the record type to be associated with this layout and select the associated grid/sub-grid.
Alternatively, scroll to the record type in the layouts tree, open the Grid/Sub-grid and right-click on it. This will allow you to add (or duplicate, edit, delete) a layout while directly simultaneously associating it with that record type.
Highlight the user groups you would like to add/remove from the layout, then click Edit. Check the box in the Show column if you would like the user group to have access to the layout. Check the default box if you would this layout to be the primary layout for the user group. Once all information is entered, click Update Records.