I have been experimenting with PowerShell scripting for the past few months.

Below is a script I have written to remove all users in a specific SharePoint group. There have been many instances where we find users have added too many individual user accounts to a specific SharePoint group. This causes many issues like search failing, performance issues, etc.

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[System.Reflection.Assembly]::Load("Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")
[System.Reflection.Assembly]::Load("Microsoft.SharePoint.Portal, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")
[System.Reflection.Assembly]::Load("Microsoft.SharePoint.Publishing, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")
[System.Reflection.Assembly]::Load("System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")

###########################
# "Enter the site URL here"
$SITEURL = "http://serverurl"

# "Name of Site group from which users have to be removed"
$SITEGROUP = "Site_Contributors"

###########################

$site = new-object Microsoft.SharePoint.SPSite ( $SITEURL )
$web = $site.OpenWeb()
"Web is : " + $web.Title

$oSiteGroup = $web.SiteGroups[$SITEGROUP];

"Site Group is :" + $oSiteGroup.Name
$oUsers = $oSiteGroup.Users

foreach ($oUser in $oUsers)
{
    "Removing user : " + $oUser.Name
    $oSiteGroup.RemoveUser($oUser)
}

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Disclaimer: The above code is provided "As Is". This is just a sample code and is not production ready.