Sorry moontide, you caught us on a 3-day weekend and we're getting caught up now. Lemme see if I can help you out.
Explore:
This area is for highlighting really great content on Virb. It's curated by us (the Virb staff) and gets updated once or more a day. It's the best of the best on Virb. If you'd like to see all the photos that members are adding to the site, you want the Public Contributions page in your hub. Here's a link: http://virb.com/hub/activity/contributions/public/photos
Groups
Currently we've only *featured* three groups in the Explore section. Again, that area is for stuff we (the Virb Staff) are showcasing as the best of the best. To find more groups, check out other users profiles and click on their Groups button to see which ones they've joined. You can also do a search and find groups on topics that interest you.
Comments
Your comment section in the hub is specifically for the comments left on your profile items (photos, videos, text, etc); and for comments you leave on other people's profile items (photos, videos, text, etc). Group comments are separate from profile-related comments. And all activity for groups is on the Activity > Groups page here: http://virb.com/hub/activity/groups/
Hope this helps!