This simple extension allows you to automatically add news users into one or more groups.
Via WPS Pro->Setup->Default Groups, enter the ID of the group, that you want new users to automatically become a member of. If you want to add them to more than one group, enter several separated by commas.
You can find the Group IDs via WPS Pro->Groups (or click on the link provided). On the screen you then see, there is a column showing the ID of each group, for example:
Provided in the Extensions Plugin.