You have to either know the name of their Armor Games account, and type in your search bar example... armorgames.com/user/HUA7XFan122396 ... and there is an add friend button. Option 2 if you see someone in the forums you can click on the link to their profile. Option 3 if you see someones comment on a game, you can click the link to their profile to add them. Those are all the options I know of.
