This simple addon allows you to choose a group that new users will automatically be added to upon registration. After installing the addon, you can set the group that new users will be added to by clicking the "Registrant Group" tab in the "Users and Groups" dashboard section. Note that this setting only applies to users who register through the front-end registration form (if you add new users via the dashboard "Add User" page, you will still be able to manually choose which group(s) the user will go into).