>>> "I believe
that was implemented so that others couldn't submit
subscription requests using your email address."
Email is insecure, being online requires the account to be logged in hence more secure therefore we can relax a bit.
If your objective is to prevent the confirmation email, and you're sending out main-group messages with subscribe links to the subgroups, how about having the links to their home pages instead, and tell folks to click on Join? Yes I know, it still doesn't address email-only preferred users, they still would have to do the extra step. But for the rest, work-wise it's easier and faster to click once to go to the group home then click Join and be done, and then they are on the subgroup's pages and can start exploring right away.
Or alternatively, if the subgroups are announcement groups, you could have a single link to the main group's "Subgroups" link in your message, where all the available subgroups are presented and the members can then join the ones they want to.