Add Users to a SharePoint Group in Bulk

Add-UserToSharePointGroup.ps1

function Add-UserToSharePointGroup
{
    param
    (
        [Parameter(Mandatory=$true)][Microsoft.SharePoint.SPWeb]$Web,
        [Parameter(Mandatory=$true)][string]$SharePointGroupName,
        [Parameter(Mandatory=$true)][string]$UserName

    )

    begin
    {
    }
    process
    {
        $group = $web.SiteGroups | ? { $_.Name -eq $SharePointGroupName }

        if( $group )
        {
            $userPrincipal = New-SPClaimsPrincipal -Identity $UserName -IdentityType WindowsSamAccountName

            $spUser = $web.EnsureUser( $userPrincipal.ToEncodedString() )
        
            if( $spUser )
            {
                $group.AddUser( $spUser )
            }
            else
            {
                Write-Error "User was not added to group: $UserName" 
            }
        }
        else
        {
            Write-Error "A SharePoint group with the name '$SharePointGroupName' was not found."
        }
    }
    end
    {
    }    
}

# get to the site 
$web = Get-SPWeb -Identity "https://sharepoint.contoso.com/sites/teamsite"

# import the list of users
$users = Import-Csv -Path "C:\Users.csv"

foreach( $user in $users )
{
    Add-UserToSharePointGroup -Web $web -SharePointGroupName "Team Site Members" -UserName $user.UserName
}