In addition to interactive way of Jim, to do it as a Script see the following example:
Names Default To Here( 1 );
// define a column group and set format of that columns in the group
dt = Open( "$SAMPLE_DATA\Big Class.jmp" );
dt << group columns( "decimals", {:height, :weight} );
For Each( {value}, dt << get column group( "decimals" ),
Print( value );
value << Format( "Fixed Dec", 15, 3 );
);
Georg