Get all groups & members for a Site Collection

Add-PsSnapin Microsoft.SharePoint.PowerShell

$siteUrl = "http://somesite/sites/collection"
$web = Get-SPWeb $siteUrl

@(foreach ($group in $web.SiteGroups) {
  foreach($user in $group.Users) {
    $usergroup = New-Object System.Object
    $usergroup | Add-Member -type NoteProperty -name GroupName -value $group.Name
    $usergroup | Add-Member -type NoteProperty -name UserName -value $user.Name

     Write-Output $usergroup
    }
}) | Export-Csv c:\userlist.csv -NoTypeInformation