BuddyPress.org

Dan-

What John was referring to was having the “Blog Tracking” component enabled in BuddyPress, not allowing users to create new blogs. If you log into WPMU as site admin, then navigate to “BuddyPress > Component Setup”, you can enable or disable any of the core BuddyPress components. By default, they are all enabled.

So, in your particular case where your users cannot create new blog posts but can only comment on blog posts, if “Blog Tracking” is disabled, then users’ comments will not show up in the activity stream. The enabling or disabling of new blog creation is a WPMU not BuddyPress feature.