Add-GroupMembership.ps1

function Add-GroupMembership
{
  [CmdletBinding(DefaultParameterSet='Pipeline')]
  param(
    [Parameter(Mandatory=$true, ValueFromPipeline=$true, ParameterSetName = 'Pipeline')]
    [object[]]$InputObject,
    
    [Parameter(Mandatory=$true, Position=0, ParameterSetName = 'Explicit')]
    [string]$User,

    [Parameter(Mandatory=$true, Position=1, ParameterSetName = 'Explicit')]
    [string]$Group
  )

  begin {
    $membershipsToAdd = @{}
  }

  process {
    if($PSCmdlet.ParameterSetName -eq 'Explicit') {
      $membershipsToAdd[$User] = $Group
    }
    else {
      foreach($record in $InputObject) {
        $membershipsToAdd[$record.Username] = $record.Groupname
      }
    }
  }

  end {
    foreach($user in $membershipsToAdd.Keys) {
      Add-UserToGroup -User:$user -Group:$membershipsToAdd[$user]
    }
  }
}