Limit User Name & Password Verification Attempts

I'm trying to write what is supposed to be a simple script that looks at a column of usernames (ColumnA) and passwords (ColumnB). I start off by prompting the user to enter their username in an input box and then I want a loop to check this value against the values in Column A. If the username exists, I want to prompt an input box for the password. If the username does not exist I need some kind of limit on the loop to prompt an input box for the user to enter a new username and password that I'll add to the last row of the data table. The loop part is kicking my butt, this is what I have so far...

Sub Main()
Set StartRange = Worksheets("16.10"). Range("A1")
username = InputBox("Please enter your username.", "Enter username", "Jdoe")
i = 1
Do While StartRange.Offset(i, 0).Value <> "username"
i = i + 1
Loop
If StartRange.Offset(i, 0).Value = "username" Then
password = InputBox("Please enter your password.", "Enter password", "password")
End If
End Sub