One of the best features of pkmn.gg is our social features which includes our Friends System.
Click on the Friends icon in the top avatar menu (on mobile, tap on your avatar and then tap friends)
Head to the Find a Friend tab and enter their pkmn.gg Username to send them a Friend Request.
You'll see a yellow notification dot on the friends icon and a notification button of new friend requests on your profile when you have pending requests. You can choose to accept or decline new friend requests.
You can customize your profile privacy in Account Settings. You can choose to have your profile visible to all or only to approved friends.