A custom SPD workflow step that automatically adds user to the correct security group, the user name can be driven from a list item or document library item that that would trigger this workflow. The user name for the user to be added to the SharePoint security
group can also be a dynamic string composed within the workflow it's self.
I'm not planning on releasing updates to this solution, but the source code is available.
Deployment steps are described here on my blog http://spforsquirrels.blogspot.com/2010/04/custom-workflow-step-to-add-user-to.html
Created by Natalya Voskresenskaya - Microsoft SharePoint MVP
My blog: http://spforsquirrels.blogspot.com