Please refer my previous article to learn more about creating NINTEX workflow.
We will be adding the user to SharePoint group by calling the web service _vti_bin/usergroup.asmx. I have created a list which has Name – Single line of text, UserLoginName – People picker and Comments – Multi line of text field.
Workflow Design:
Web Request Action
Soap Action
Set Field in Current Item
Result
The specified user is added to the specified group successfully.