You method seems to be working. Now this is a good place for learning. Now when you executed the SWUS, you see the message the work item was started. Immediately after that click the "Workflow log" button. This will bring you to the log, and if you have changed your personal workflow settings, you should see the technical log. There you should be able to see the container of the work item. If in the container the TAB container element includes the user IDs, then you know that your task is working. (The TAB in the starting screen of SWUS will not be populated.)
Once the task is working, its time to create a new step to the workflow. (Well, you have it already.) It seems that you are binding the TAB from the task container to a TAB1 in the workflow container. That means the user IDs should be in TAB1. Now open the email sending step. Do the binding from the workflow container (TAB1) to the email sending step/task container element ADDRESSSTRINGS. (From left to right). Actually most probably there is still one problem. You should have prefix US in front of the user ID - the email sending step is expecting them in that format. You might need to modify your method.
Regards,
Karri