Add-PSSnapin Microsoft.SharePoint.PowerShell
  
#Read CSV file
$CSVData = Import-CSV -path "C:\UserList.csv"

#Set 
$WebUrl = "http://sitecollection/sites/PM"
$GroupName = "PM Members"

#Get the Web
$web=Get-SPWeb $WebUrl
#Get the SharePoint Group
$Group= $web.Groups[$GroupName]

#Loop every Row in the CSV
foreach ($Row in $CSVData)
{
    #Get UserName
    $userName = $row.UserName

    #Add User to the site collection
    $user = $web.EnsureUser($UserName)
 
    #Add User to the Group
    $group.AddUser($user)

    Write-Host "Added User: " $row.UserName -ForegroundColor Green

}