Creating a Contact Group within Outlook

Click the "People" icon, either on the left menu or on the bottom left (Depending on your Outlook version)

Click "Home" along the top and then "New Contact Group". In older Outlook versions it may be in the dropdown box next to "New Contact"

Give the group a name and add members using the "Add members" button at the top (And then from address book)

When finished click "Save and close"

To email this group go to the people section of Outlook, search for the group name and then click "Email" along the top