Enable the Follow/Unfollow functionality not just at the All Groups-All Topics central hub, but also anytime one is in topic (or maybe also message) view in any group; or at least enable it based on whether someone has set a specific group's subscription to Following-Only or not.

Alternatively, upgrade the All Groups-All Topics screen with various sorts and filters to enhance functionality and usability; the way that screen is naked now, makes using it cumbersome to anyone but folks with exclusive Following-Only subscription setting and/or very few groups.