Create an AD user from a SQL Server trigger
What @Mike_Fal said. This is a huge gaping security hole. Not only does SQL need xp_cmdshell, but it will need Domain Admin rights. I also agree with the other sentiments around not using a trigger.

You can still tie it in with your existing process and make it database driven with the security table you are using. I'd probably write a separate, asynchronous process to read from the security table and add the AD users that way. This is a great job for PowerShell; connecting multiple systems and technologies.