I currently have a few bundles of feeds that I'm sharing with friends. I'm not sure how to add more feeds to existing bundles.
Go to "Browse for Stuff" then "view your bundles". Click the "Edit" link.
Drag subscriptions you want to add from your subscription list on the left and drop in the main box. Drag subscriptions that you want to remove from the main box and drop in the "Drag a subscription here to remove it from the bundle" box.