Append to CSV in PowerShell?

I'd like to find a way to append to an outputted csv file in Powershell.  Out-File has too much wasted space and duplicate lines.  Here's the script I'm trying to run:

$a=Get-Content users.txt
foreach ($i in $a)
{
    get-user $i | select displayname,samaccountname | Export-Csv output.csv -notype
}

I can't seem to find anything when dealing with an input file...  Thx.