Add and delete user in table

Hi, guys. I am doing a system which is allowed admin to control user.

The ID is = admin

The pass is = 6649481

Can anyone help me to check for the add and delete part.

Actually, it can work but I would like to make it not repeat is allowed but this only work on the index "0" then from index 1 it will not stop user to add repeated ID.

Delete part if I were removing the loop will be fine and work fine but it can only delete from the top. It's like First in first out concept but I wish to make it like according by the ID given to delete from table.