What environment do you use? SharePoint online or SharePoint server?
SharePoint online :
You can add multiple users to Office 365 in the Microsoft 365 admin center
1.Sign in to Office 365 with your work or school account.
2.In the admin center, choose Users > Active users.
3.Select Add multiple users.
Add several users at the same time to Office 365
I recommend you use powershell, you can refer to the commands provided in the two threads below:
Add multiple users, groups and groups permission from *csv file to SharePoint 2013 using powershell
Add user to SharePoint 2010 group using powershell
In addition, if you know the users email addresses, you can add them in bulk, delimited by a semicolon.