SharePoint: populate security groups from CSV

#Group,P1,P2,P3,P4,P5,P6,P7,P8,P9,P10
#Initiators,John Doe,Nicholas Smith,Jonathan Smith,,,,,,,
#Reviewers,Nicholas Smith,Jonathan Smith,Dave Smith,Tosha Doe,,,,,,

# Read CSV and populate groups

$webUrl = "http://acme.com/sites/acme/PremiumPayment2"
$csvPath = "C:\Temp\20150427\UserGroupSetup.csv"

$csv = Import-Csv -Path $csvPath

$w = get-spweb $webUrl

function EmptyGroup($g){
    "Cleaning group: $($g.Name)"
    while($g.Users.Count -gt 0){
        $u = $g.Users[0]
        "Removing user: $($u.DisplayName)"
        $g.RemoveUser($u)
    }
}

foreach($gg in $csv){
    $gName = $gg.Group
    "Checikng if exists: `"$($gName)`""
    $g = $w.SiteGroups[$gName]
    if($g -eq $null){
        throw "Group `"$($gName)`" not found"
    }
}

$uniqueNames = new-object -TypeName "System.Collections.Generic.List[string]"
foreach($gg in $csv){
    $gName = $gg.Group
    for($i = 1; $i -le 10; $i++){
        $name = $gg."P$($i)"
        if($name -ne $null -and $name -ne ""){
            if(-not $uniqueNames.Contains($name)){
                $uniqueNames.Add($name)
            }
        }
    }
}

$allUsers = get-spuser -Web $w.Url
$userDic = new-object -TypeName "System.Collections.Generic.Dictionary[string,Microsoft.SharePoint.SPUser]"

"Checikng users"

foreach($name in $uniqueNames){
    $user = $allUsers |?{$_.DisplayName -eq $name }
    if($user -eq $null){
        throw "User not found: $($name)"
    } else {
        if($($user.GetType().Name) -eq "SPUser"){
            $userDic.Add($name, $user)
        } else {
            $acmeUser = $user|?{$_.UserLogin.StartsWith("ACME\")}
            $userDic.Add($name, $acmeUser)
        }
    }
}

foreach($gg in $csv){
    $gName = $gg.Group
    "Processing group: `"$($gName)`""
    $g = $w.SiteGroups[$gName]
    EmptyGroup $g
    for($i = 1; $i -le 10; $i++){
        $name = $gg."P$($i)"
        if($name -ne $null -and $name -ne ""){
            $user = $userDic[$name]
            $g.AddUser($user)
        }
    }
}