Please note: you need to be the group owner in order to modify the members of the group.
1. Go to the Address Book in Outlook as highlighted below.
2. This will open the following Window.
3. Search for the group as highlighted.
4. Right click on the group name and select Properties.
5. Click the Modify Members... button.
6. Click either the Add or Remove button as required.
7. If adding a user to the group you will have to search for them using the Add Users screen.
8. When the user has been found, select them first, click on Add ->, then on OK.