Once I had to add about 3 thousand users to SharePoint group. Usually when I have to add about 100 users, I use Excel function 'CONCATENATE' to add user login and ";" separator to generate a list of users. And then I manually enter them. But 3 thousand users – it's too much for manual.

Below I write down PowerShell script for SharePoint to add users to SharePoint group.

I have left comments in script for easier to find params for script. This script takes a text file with logins (each login on each line) and adds user to SharePoint group. As a result you'll have a log-file with statistics.

#PowerShell script to add users to SharePoint group from txt file 
if ((Get-PSSnapin "Microsoft.SharePoint.PowerShell" -ErrorAction SilentlyContinue) -eq $null) 
{   Add-PSSnapin Microsoft.SharePoint.PowerShell } 

#set location for data files 
$loc = 'C:\Tools\SPScripts' 
#read array of string to $Users array from users.txt file 
$Users = Get-Content "$loc\users.txt"         
#set domain for users 
$domain = 'DomainName\' 
#set group name 
$groupName = 'Developers' 
#set location of log file 
$logFile = $loc + '\logfile.txt' 

$siteCollUrl = 'http://spdev/' 
$spwebUrl = $siteCollUrl + 'devsite/'
$web = Get-SPWeb -identity $spwebUrl 

$group = $web.Groups | where {$_.Name -eq "$groupName" } 
Write-Host $group 

$header = "Adding users to $groupName"

$header | Out-File $logFile

$total = 0
$success = 0
$errors = 0
 
foreach ($User in $Users) { 
        $total++ 
        $domainUser = $domain + $User 
        try { 
                #If user is not in site collection, create SPUser 
				$SPUser = $web.EnsureUser($domainUser) 
                $group.AddUser($SPUser) 
                $msg = "Success        $User is added" 
                $msg | Out-File $logFile -Append 
				$success++
        } 
        catch { 
                $errorMessage = "Error        $User        $_.Exception.Message" 
                $errorMessage | Out-File $logFile -Append 
				$errors++
        }         
} 


$result = "----------------
Total: $total, Success: $success, Errors: $errors
"

Write-Host $result
$result | Out-File $logFile -Append