Search in DataGridView (without database or query binded)

Good day;

i want to add row in blank DataGridView by DataTable, this adding should follow 1 rule (in pseudo code)::

"IF item before added, update 'Number' cell by refrenced TextBox (DataGridView Cell's: FName, LName, Number)

ELSE add new row"

my form: http://up2www.com/uploads/152eC-.png

but there are two problems:

1. first row could not be added by below code (no error). (first row should be add by another button or default value)

2. Compare to find a existent row similar to the row being added, played only the first row of the DataGridView.

my code: attached

Please help me by code