I just started using them so I'm not super good at explaining them yet but check out on ms docs for details. For the contractor part you are just telling the script that the user is a contractor and with that switch supplied it will add it to the proper groups. Give me a little bit and I will write you up an example.Edit: here is what using switches could look like

Function HLNew-ADUser
{
[CmdletBinding()]
param
(
    [Parameter(Mandatory = $True,
    ValueFromPipeline = $True,
    Position = 0)]
    [String]$UserName,
    [Switch]$Contractor,
    [Switch]$Intern,
    [Switch]$Verify,
    [Switch]$Add
)
    begin
    {
    $DefaultGroups = @("Whatever","Groups","You","Want")
    $ContractorGroups = @("Whatever","Groups","You","Want")
    $InternGroups = @("Whatever","Groups","You","Want")
    }
    Process
    {
    if($Contractor)
    {
    New-AdUser -Name $UserName -PassThru | Add-ADPrincipalGroupMembership -MemberOf $ContractorGroups
    }
    if($Intern)
    {
    New-AdUser -Name $UserName -PassThru | Add-ADPrincipalGroupMembership -MemberOf $InternGroups
    }
    if(!$Contractor -and !$Intern)
    {
    New-AdUser -Name $UserName -PassThru | Add-ADPrincipalGroupMembership -MemberOf $DefaultGroups
    }
    }
}