I've sent a link out to https://[list-name].groups.io/g/[sub-group-name]? for people who are not yet groups.io members to join a sub-group. They are not yet members of the main group. The process throughout (e.g., web page, email confirmation) all refer to the sub-group name, but for a message saying they must first join the main group (though with no information about how to do so.) When they complete the process by confirming their email and adding a password, they are actually subscribed to the main group, and not the sub-group. But, with no notice to them that they've not actually been added to the sub-group, just the main group. So, they think they are done.
It would be a better user experience in this scenario (request to join sub-group by person who is not yet in main group) if they were added to both in a single operation.
