How to add a large number of users to a SharePoint site?

Yes. Use SharePoint groups plus either the UI (for smaller batches) or PowerShell with CSV (for thousands of users).

1. Add users via the SharePoint site (good for smaller batches)
2. On the target SharePoint site, open the Site Actions menu and select Site Settings.
3. Under Users and Permissions, select People and Groups.
4. In the left navigation, select Groups and choose the visitors group (for example, [Site Name] Visitors).
5. On the menu bar, select the arrow next to New, then select Add Users.
6. In Users/Groups, paste or type user accounts separated by semicolons.
   • For domain accounts, use DOMAIN\alias format and select Check Names.
7. Select OK to add them as visitors.

This works but is cumbersome for thousands of users.

2. Bulk-add thousands of users with PowerShell and CSV (recommended)

For ~3,800 users, use SharePoint Online PowerShell with CSV files:

1. Create a CSV file (for example, Users.csv) with headers:

      Group,LoginName,Site
   

   Each row defines which user goes into which group on which site, for example:

      Site Visitors,******@contoso.com,https://tenant.sharepoint.com/sites/YourSite
      Site Visitors,******@contoso.com,https://tenant.sharepoint.com/sites/YourSite
   

2. (Optional) If the visitors group does not exist yet, create another CSV (for example, GroupsAndPermissions.csv) with headers:

      Site,Group,PermissionLevels
   

   Example row:

      https://tenant.sharepoint.com/sites/YourSite,Site Visitors,Read
   

3. Use PowerShell to create groups (if needed) and add users:

      Import-Csv C:\O365Admin\GroupsAndPermissions.csv | ForEach {
          New-SPOSiteGroup -Group $_.Group -PermissionLevels $_.PermissionLevels -Site $_.Site
      }
      
      Import-Csv C:\O365Admin\Users.csv | ForEach {
          Add-SPOUser -Group $_.Group -LoginName $_.LoginName -Site $_.Site
      }
   

This approach is designed specifically to automate adding large lists of users and groups to SharePoint sites and is appropriate for thousands of site visitors.

References:

• Add users to the Search Center site
• Manage SharePoint users and groups with PowerShell
• Create SharePoint sites and add users with PowerShell