For adding a row from a button ouside the grid. I m using infragistics wed image button. you can use normal asp button also.

 <igtxt:WebImageButton ID="btnAddItem" runat="server" CausesValidation="False" Text="New Item"
                                                ToolTip="Add new Item" AutoSubmit="false">
                                                <Appearance>
                                                    <Image Url="./Images/img_add.png" />
                                                    <ButtonStyle Cursor="Hand" Font-Names="Arial" Font-Size="8pt">
                                                    </ButtonStyle>
                                                </Appearance>
                                                <RoundedCorners RenderingType="Disabled" />
                                                <HoverAppearance>
                                                    <ButtonStyle Font-Underline="True">
                                                    </ButtonStyle>
                                                </HoverAppearance>
                                                <Alignments VerticalImage="Bottom" />
                                                <ClientSideEvents Click="AddRow" />
                                            </igtxt:WebImageButton> 

I am adding a row to the  grid and setting the value of the two column in the grid.

function AddRow(oButton, oEvent) {
     igtbl_addNew("dgGrid", 0);
     var grid = igtbl_getGridById("dgGrid");
     var activeRow = grid.getActiveRow();
     activeRow.getCellFromKey("Col1").setValue(0);  
     activeRow.getCellFromKey("col2").setValue("ABC TEXT");
     activeRow.endEditRow();   

     oEvent.needPostBack = false;
}

For deleting the from the button outside:

How will you identify that which row you want to delete ?

var row = // row you want to delete based on your condition and requirement

row.deleteRow();