To add friends using the Geo-Filter, enter a private lobby on your desired game and disable your device by un ticking the "Enable" checkbox. Now invite your friend into the lobby. Look at your Geo-Filter map - you should see a big circle appear. This will be your friend!
Click on the circle, and then in the handle box rename it to whatever you like, e.g. My friend Alfie. Drag the quality bar to 100% and then click update.
You will see a circle appear in the green section of the Deny & Allow feature. You can click on this circle to edit the settings you input at any time.
More information on Deny & Allow a game host.