do
{
$username = Read-host "Please enter an existing username"
} while ($username -eq "" -or (Get-ADUser -Filter {Name -eq $username}) -eq $null)
This is case insensitive though. This one might work better:
do
{
$username = Read-host "Please enter an existing username"
} while ($username -eq "" -or (Get-ADUser -Filter "Name -like `"$username`"") -eq $null)