I would agree that it would be nice to have a single page where new subscribers can join the group and set their password for web access. Right now, it's two separate actions and a stumbling block for many of my subscribers also. The invite email could have 2 options: 1) Reply to subscribe with email access only -and- 2) Click link to subscribe on the web and setup web access to the group (which goes to new page where subscriber joins and sets password).
I really like that it has FB/Google integration but that needs more advanced web skills and presumes some prior knowledge before subscribing (using the gmail address instead of an ISP address for your subscription which you don't know you can do until after you subscribe. Yes, you can change it but that's even more involved.)