I never put any of them in groups unless I get a specific invite on those images that get into Explore - I'll usually just click "yes" on those unless it looks like a group I really don't want to be represented in. Way back when I was first on Flickr, I joined a few groups and tried to put my photos in them, but it was just one more thing to keep track of and it fell by the wayside very quickly. I make my own albums for a particular trip or event (and then share those albums with family and friends), but I very very rarely mess with Flickr's own groups.
Groups, along with tags, geotags, titles, descriptions, and exif data, are a very useful filter when researching or trying to get a bit of inspiration from different cameras, lenses, styles, genres, possible holiday destinations, etc. Even when searching through my own images I find all of that information very helpful, let alone when searching everyone else's!