Join (using an email address never before registered)
1) After Email Delivery asks for email address and password. It isn't clear whether filling in these is to login or to register.
Evidently either, at need. At least, filling it in when I had no prior registration gave me "You are now subscribed to the group. Once you reply to the confirmation email we just sent you, you will be able to participate in the group."
This phrase is awkward:
"To join any of the groups on Groups.io, you must log in, if you're already a registered Groups.io user, or register, if you're new."
"To join any groups you must log in below. If you haven't already registered with Groups.io you can register now by entering your email address below and choosing a new password that you will use to log in to Groups.io."
2) Very clever that you apparently picked up my "Full name" from the Display name field of the confirmation email sent back during registration. This item started out as noticing that there was no place on the Join form to enter my screen name.