I 100% agree that the terms should be defined in the help pages, but I also think there should also be a short explanation in the initial email. Maybe something like this (suggestions in bold and/or strikethrough):
Thank you for your interest in the [Group email address replace group email address with group title] group at Groups.io. If you did not request or do not want to join [Group email address title], please ignore this message.
The next step is for you to confirm your email address to activate your account with Groups.io. This will not activate your membership with the [group title] group if they require your membership to be approved by a real person. You will receive an additional email from them with more details once your email address has been confirmed.
If you want to use the resources and read messages on the [group title] website, please click on the link below to confirm your email address, set up a password, and choose other subscription settings:
[Comfirm account link]
Thank you for confirming your account. You will now receive messages from the groups you are subscribed to. If any of the groups you subscribed to require approval before joining, you will be notified when that happens.
The Groups.io Team
While this is okay, I would again suggest there be a different message go out for restricted groups, so they know what to expect. And as J suggested, the group title be added to the message as well. Also, the message says "You will now receive messages from the groups you are subscribed to" while is true, they will only be receiving this message while confirming their email address from their first group. So maybe a better way to say it would be:
You will now receive messages from the [groups title], as well as any other groups to which you subscribe.