How do you add a friend?

I can't find any way to add or invite a friend on here or on the mobile app. They already have an account.

You can't really invite people to friend. You can ask them to follow you though. But there isn't an automatic way to do it. I would say, send them a PM and ask them to follow.
Exactly what I would say.
If you follow them, they may get the hint and follow back.