Tap more, then go to settings, then profile and scroll down to User ID. Once you set a User ID you also have to make sure that the Add by ID is turned on.
If you don't want to set an ID you can, click more, then add friends, go to QR Code, My QR code at the bottom, tap the down arrow in the top right then either share or send by email. Once you do that it will email or text you a copy of your QR code as well as a link that people can use to add you as a friend. See my example, minus the QR attachment below.