To add a new user to a specific user group in SharePoint, follow the steps below:

1. Login to your SharePoint Online site with your administrator account.
2. Navigate to the Settings by selecting Site Settings in the left menu.
3. Under Site Settings, click on People and groups.
4. Find the group you would like to add a user to and select it, then click on New.
5. Choose Add existing users.
6. Type in the name of the user, then select their name when it appears.
7. Click on Share. The user will be added to the group.