Unfortunately the "Add User by Name" Alchemy plugin doesn't work anymore in SDL Tridion Sites 9. Is there another way to install it or an alternative? I urgently need this plugin because the standard interface to add users is getting a timeout when I try to add a users
You could create the users using theCore Service:
UserData user = client.Create(user, new ReadOptions()) as UserData;
Jan has done something similar in the past (for Tridion 2013), so you should be able to use that as a starting point: http://sharedbyjan.blogspot.com/2014/03/i-was-at-new-project-this-week-creating.html
Isn't the problem that Alchemy itself doesn't support Sites 9 yet?
Anyway, the Tridion PowerShell Modules have an easy alternative:
New-TridionUser -Name "DOMAIN\UserName" -Description "Full Name of User"