Thread:Trexmaniac511/@comment-32769624-20190318171224/@comment-31620988-20190531073238

Click "friends" on the top left side (it's on the list with Activity, Library, etc), then you'll see a green button on the top right "Add Friend". Paste the ID of the user in here you want to add. While the UI was updated since, this video can still guide you to where the buttons are:  https://www.youtube.com/watch?v=0IOcELLyWGo

Hope that helps!