Help Page. To be fair it is pretty clear that "user" is synonymousOnly in a group context. In an account context "user" is mora appropriate because the person may not be a member of any groups.
The uses in the Help page look correct to me, except possibly in the case of the Invite section. There the invitee may not (yet) qualify as even a user of Groups.io, so perhaps:
"We will send an email invitation to each /address/. To join the group, each /person/ simply has to click on the link within the invitation, or reply to the invitation."