Friends on Slopes work a bit differently than other social networks (by design!). Without public profiles, usernames, or anything like that, Slopes relies on you already being able to talk to your friend via some other service so you can send them a link. This goes a long way to avoid creepy strangers sliding into your DMs, bots, harassment, and more.
Adding a friend is still super simple, though.
Hit the + button in the top right of the Friends tab and use the share sheet to pick how you want to send the friend link. On the other end, all your friend has to do is tap that link to open up Slopes and confirm that they want to friend you.