You must be the device owner to add users.
Go to Settings > System > Users. If needed turn Allow multiple users on 

You can remove yourself as a user. You must be the device owner to remove other users.
If you're a user who borrowed a phone:
Go to Settings > System > Users.
Or from quick settings, tap 

If you're the owner:
Go to Settings > System > Users.
Or from quick settings, tap 
