If you ask a friend to create a SG account so that they can join your buddy read, it will be awkward. Your friend won't know about SG, so they are likely to create their account with the default security settings -- thus you won't be able to add your friend to the buddy read, because their username will not appear in the search. To get them into the buddy read, you'll have to ask them to change their settings (if you can even figure out the problem yourself). By this time you start to lose their attention -- but even if they do it, it still may not work. In my case I wasn't able to add my sister after we went through this (which seems like a bug, but I don't know if she followed my directions), so finally she created her own buddy read and invited me instead.

Buddy Reads is a great feature in itself, but also a great way to get people to join SG, so I think this flow needs to be smoother. I don't have a clear suggestion for how to do that however. It might be as simple as changing the default settings. Or maybe there could be a flow for inviting friends to buddy reads who don't have accounts, which sets up their account to allow the invite to happen, or auto-joins the buddy read for them if they click accept.